REESE  LIBRARY 

OF  THK 

UNIVERSITY  OF  CALIFORNIA. 

Class 


HEAT 


CONSIDERED   AS 


A  MODE    OF    MOTION: 


BY 


JOHN  TYNDALL,  F.R.S.,  &c. 

PBOFESSOR    OF    NATURAL    PHILOSOPHY    IN    THE    BOYAL    INSTITUTIOK 
AND    IN    THE    EOYAL    SCHOOL    OF    MINES. 


FROM  THE  SECOND  LONDON  EDITION  REVISED,   WITH  ADDITIONS  EMBRACING  TUB 
AUTHOIHa  LATEST  RESEARCHES. 

rtp£  LI.S&,; 


r.  .^. 


D.    APPLETON    &    COMPANY, 

90,  92  &  94  GRAND  STREET. 
1869. 


PEE  FA  0  E. 


IK  the  following  Lectures  I  have  endeavoured  to 
bring  the  rudiments  of  a  new  philosophy  within  the 
reach  of  a  person  of  ordinary  intelligence  and  culture. 

The  first  seven  Lectures  of  the  course  deal  with 
thermometric  heat ;  its  generation  and  consumption  in 
mechanical  processes ;  the  determination  of  the  me- 
chanical equivalent  of  heat ;  the  conception  of  heat  as 
molecular  motion ;  the  application  of  this  conception 
to  the  solid,  liquid,  and  gaseous  forms  of  matter ;  to 
expansion  and  combustion  ;  to  specific  and  latent  heat ; 
and  to  calorific  conduction. 

The  remaining  five  Lectures  treat  of  radiant  heat ; 
the  interstellar  medium,  and  the  propagation  of  motion 
through  this  medium  ;  the  relations  of  radiant  heat  to 
ordinary  matter  in  its  several  states  of  aggregation ; 
terrestrial,  lunar,  and  solar  radiation ;  the  constitution 
of  the  sun  ;  the  possible  sources  of  his  energy ;  the  re- 
lation of  this  energy  to  terrestrial  forces,  and  to  vege- 
table and  animal  life. 

My  aim  has  been  to  rise  to  the  level  of  these  ques- 
tions from  a  basis  so  elementary,  that  a  person  possess- 
ing any  imaginative  faculty  and  power  of  concentration, 
might  accompany  me. 

Z  5  1 9~  5 


IV  PREFACE. 

Wherever  additional  remarks,  or  extracts,  seemed 
likely  to  render  the  reader's  knowledge  of  the  subjects 
referred  to  in  any  Lecture  more  accurate  or  complete, 
I  have  introduced  such  extracts,  or  remarks,  as  an  Ap- 
pendix to  the  Lecture. 

For  the  use  of  the  Plate  at  the  end  of  the  volume, 
I  am  indebted  to  the  Council  of  the  Eoyal  Society  ;  it 
was  engraved  to  illustrate  some  of  my  own  memoirs  in 
the  i  Philosophical  Transactions.'  For  some  of  the 
"Woodcuts  I  am  also  indebted  to  the  same  learned  body. 

To  the  scientific  public,  the  names  of  the  builders 
of  this  new  philosophy  are  already  familiar.  As  ex- 
perimental contributors,  Rumford,  Davy,  Faraday,  and 
Joule,  stand  prominently  forward.  As  theoretic  writers 
(placing  them  alphabetically),  we  have  Clausius,  Helm- 
holtz,  Kirchoff,  Mayer,  Eankine,  Thomson  ;  and  in  the 
memoirs  of  these  eminent  men  the  student  who  desires 
it,  must  seek  a  deeper  acquaintance  with  the  subject. 
MM.  Regnault  and  Seguin  also  stand  in  honourable  re- 
lationship to  the  Dynamical  Theory  of  Heat,  and  M. 
Yerdet  has  recently  published  two  lectures  on  it, 
marked  by  the  learning  for  which  he  is  conspicuous. 
To  the  English  reader  it  is  superfluous  to  mention  the 
well-known  and  highly-prized  work  of  Mr.  Grove. 

I  have  called  the  philosophy  of  Heat  a  new  philoso- 
phy, without,  however,  restricting  the  term  to  the  sub- 
ject of  Heat.  The  fact  is,  it  cannot  be  so  restricted ; 
for  the  connection  of  this  agent  with  the  general  ener- 
gies of  the  universe  is  such,  that  if  we  master  it  per- 
fectly, we  master  all.  Even  now  we  can  discern,  though 
but  darkly,  the  greatness  of  the  issues  which  connect 
themselves  with  the  progress  we  have  made — issues 
which  were  probably  beyond  the  contemplation  of 


PEEFACE.  V 

those,  by  whose  industry  and  genius  the  foundations 
of  our  present  knowledge  were  laid. 

In  a  Lecture  on  the  *  Influence  of  the  History  of 
Science  on  Intellectual  Education,'  delivered  at  the 
Royal  Institution,  Dr.  Whewell  has  shown  { that  every 
advance  in  intellectual  education  has  been  the  effect  of 
some  considerable  scientific  discovery,  or  group  of  dis- 
coveries.' If  the  association  here  indicated  be  invari- 
able, then,  assuredly,  the  views  of  the  connection  and 
interaction  of  natural  forces — organic  as  well  as  inor- 
ganic— vital  as  well  as  physical — which  have  grown, 
and  which  are  to  grow,  out  of  the  investigation  of  the 
laws  and  relations  of  Heat,  will  profoundly  affect  the 
intellectual  discipline  of  the  coming  age. 

In  the  study  of  Nature  two  elements  come  into 
play,  which  belong  respectively  to  the  world  of  sense 
and  to  the  world  of  thought.  We  observe  a  fact  and 
seek  to  refer  it  to  its  laws, — we  apprehend  the  law,  and 
seek  to  make  it  good  in  fact.  The  one  is  Theory,  the 
other  is  Experiment ;  which,  when  applied  to  the  ordi- 
nary purposes  of  life,  becomes  Practical  Science.  Noth- 
ing could  illustrate  more  forcibly  the  wholesome  inter- 
action of  these  two  elements,  than  the  history  of  our 
present  subject.  If  the  steam-engine  had  not  been  in- 
vented, we  should  assuredly  stand  below  the  theoretic 
level  which  we  now  occupy.  The  achievements  of 
Heat  through  the  steam-engine  have  forced,  with  aug- 
mented emphasis,  the  question  upon  thinking  minds — 
4  "What  is  this  agent,  by  means  of  which  we  can  super- 
se'de  the  force  of  winds  and  rivers — of  horses  and  of 
men?  Heat  can  produce  mechanical  force,  and  me- 
chanical force  can  produce  Heat ;  some  common  quality 
must  therefore  unite  this  agent  and  the  ordinary  forms 


VI  PREFACE. 

of  mechanical  power.'  This  relationship  established, 
the  generalising  intellect  could  pass  at  once  to  the  other 
energies  of  the  universe,  and  it  now  perceives  the  prin- 
ciple which  unites  them  all.  Thus  the  triumphs  of 
practical  skill  have  promoted  the  developement  of  phi- 
losophy. Thus,  by  the  interaction  of  thought  and  fact, 
of  truth  conceived  and  truth  executed,  we  have  made 
our  science  what  it  is, — the  noblest  growth  of  modern 
times,  though  as  yet  but  partially  appealed  to  as  a 
source  of  individual  and  national  might. 

As  a  means  of  intellectual  education  its  claims  are 
still  disputed,  though,  once  properly  organised,  greater 
and  more  beneficent  revolutions  await  its  employment 
here,  than  those  which  have  already  marked  its  appli- 
cations in  the  material  world.  Surely  the  men  whose 
noble  vocation  it  is  to  systernize  the  culture  of  England, 
can  never  allow  this  giant  power  to  grow  up  in  their 
midst  without  endeavouring  to  turn  it  to  practical  ac- 
count. Science  does  not  need  their  protection,  but  it 
desires  their  friendship  on  honourable  terms  :  it  wishes 
to  work  with  them  towards  the  great  end  of  all  educa- 
tion,— the  bettering  of  man's  estate.  By  continuing  to 
decline  the  offered  hand,  they  invoke  a  contest  which 
can  have  but  one  result.  Science  must  grow.  Its  de- 
velopement is  as  necessary  and  as  irresistible  as  the 
motion  of  the  tides,  or  the  flowing  of  the  Gulf  Stream. 
It  is  a  phase  of  the  energy  of  Nature,  and  as  such  is 
sure,  in  due  time,  to  compel  the  recognition,  if  not  to 
win  the  alliance,  of  those  who  now  decry  its  influence 
and  discourage  its  advance. 

ROYAL  INSTITUTION,  February  1863. 


CONTENTS. 


LECTURE  I. 

Introduction— Description  of  Instruments— The  Thermo-electric  Pile  and  the  Gal- 
vanometer—Heat and  Cold  indicated  by  the  Deflection  of  a  Magnetic  Needle 
—Heat  generated  by  Friction,  Compression,  and  Percussion— "Waterfalls  gen- 
erate Heat — Friction  of  Railway  Axles — The  Force  necessary  to  heat  the  Axles 
is  -withdrawn  from  the  urging  Force  of  the  Engine — Meteorites  probably  ren- 
dered incandescent  by  Friction  against  Air — Rumford's  Experiments  on  tho 
Heat  excited  by  Friction— Water  boiled  by  Friction— Consumption  of  Heat 
when  compressed  Air  is  suffered  to  expand— Action  of  a  Current  of  Air  when 
urged  by  a  bellows  against  the  Face  of  the  Thermo-electric  Pile  .  PAGE  13 

APPENDIX   TO   LECTURE   I. 

Mode  of  constructing  a  Thermo-electric  Pile — Mode  of  constructing  a  Galvanome- 
ter— Mode  of  rendering  Needles  astatic — Experiments  on  the  Magnetism  of 
Galvanometer  Coils,  and  Mode  of  avoiding  this  Magnetism  ...  SO 

LECTURE  II. 

The  Nature  of  Heat— the  Material  Theory  supposes  it  to  be  a  Bubtle  Fluid  stored 
up  in  the  inter-atomic  Spaces  of  Bodies— The  idea  of  'Capacity'  for  Heat 
originated  in  this  way— The  Dynamical  Theory  supposes  Heat  to  be  a  Motion 
of  the  ultimate  Particles  of  Bodies — Rumford's  and  Davy's  Views — Davy's 
Fusion  of  Ice  by  Friction — Bearing  of  the  Experiment  on  the  Material  Theory 
—Heat  and  Light  developed  by  the  Compression  of  Air— Ignition  of  Bisulphide 
of  Carbon  Vapour  in  Fire  Syringe — Thermal  Effects  of  Air  in  Motion — Con- 
densation of  Aqueous  Vapour  by  the  Rarefaction  of  Air — Machine  of  Schem- 
nitz— Deportment  of  a  Conductor  between  the  Poles  of  a  Magnet— Apparent 
Viscosity  of  the  Magnetic  Field — The  Conductor  encounters  Resistance  to  its 
Motion — A  Conductor  swiftly  rotating  is  struck  motionless  when  the  Magnet 
is  excited — When  tho  Conductor  is  compelled  to  rotate  Heat  is  generated — 
Fusion  of  an  Alloy  by  this  Heat — Measurement  of  the  Amount  of  Heat  gen- 
erated by  a  given  Expenditure  of  Force— Dr.  Mayer  and  Mr.  Joule— The  Me- 
chanical Equivalent  of  Heat — Definition  of  the  Term  'Foot-pound'  —  Heat 
developed  increases  as  the  height  of  the  fall,  and  is  proportional  to  the  Square 
of  the  Velocity — Calculation  of  Heat  generated  by  the  impact  of  Projectiles — 


Vlll  CONTENTS. 

Heat  equivalent  to  the  Stoppage  of  the  Earth  in  its  Orbit— Heat  equivalent  to 
the  Falling  of  the  Earth  into  the  Sun — Preliminary  Statement  of  Meteoric 
Theory  of  the  Sun's  Heat — Analynis  of  Combustion— Ignition  of  Diamond- 
Its  Combustion  in  Oxygen  due  to  the  Showering  of  the  Atoms  of  the  Gas 
against  the  Surface  of  the  Diamond— Structure  of  Flame— Candle  and  Gas 
Flames — Combustion  on  Mont  Blanc — The  Light  of  Flames  is  materially 
diminished  by  the  Rarefaction  of  the  Air,  though  the  Quantity  of  Combustible 
Matter  consumed  remains  the  same— Frankland's  Experiments— All  Cases  of 
Combustion  are  duo  to  the  Collision  of  Atoms  which  have  been  urged  together 
by  their  mutual  Attractions PAGE  37 

APPENDIX  TO   LECTURE   II. 

Extracts  from  the  Twentieth  Aphorism  of  the  '  Novum  Organum  '—Abstract  of 
Count  Rumford's  Essay  entitled  '  An  Enquiry  concerning  the  Source  of  the 
Heat  which  is  excited  by  Friction  '—Note  on  the  Compression  of  Bisulphide 
of  Carbon  Vapour f  ,  .  67 

LECTURE  III. 

Expansion  of  Bodies  by  Heat— Liberation  of  Particles  from  the  thrall  of  Cohesion 
—The  Liquid  and  Gaseous  States  of  Matter  denned— Illustrations  of  the  Ex- 
pansion of  Air  by  Heat— Ascent  of  Fire  Balloon— Gases  expand  by  a  constant 
Increment  for  every  Degree  above  32°  Fahr. — Coefficient  of  Expansion — Heat- 
ing of  Gas  at  a  constant  Pressure — Heating  of  Gas  at  a  constant  Volume — In 
the  former  case  Work  is  done  by  the  Gas— In  the  latter  case  no  "Work  is  done 
—In  the  former  case  an  Excess  of  Heat  equivalent  to  the  Work  done  must  be 
imparted— Calculation  of  the  Mechanical  Equivalent  of  Heat— Mayer  and 
Joule's  Determinations— Absolute  Zero  of  Temperature— Expansion  without 
Refrigeration — Expansion  of  Liquids— Exceptional  Deportment  of  Water  and 
Bismuth— Energy  of  Atomic  Forces— Pyrometers— Strains  and  Pressures 
superinduced  by  sudden  Cooling— Chilling  of  Metallic  Wires  by  Stretching- 
Heating  of  India-rubber  by  Stretching— Contraction  of  stretched  India-rubber 
by  Heat 74 

APPENDIX   TO   LECTURE  III. 

Further  Remarks  on  Dilatation— Linear,  Superficial,  and  Cubic  Coefficients  of 
Expansion— The  Thermometer  —  Extracts  from  Sir  Humphry  Davy's  First 
Memoir,  entitled  '  Heat,  Light,  and  the  Combinations  of  Light '  .  , .  106 

LECTURE  IV. 

Vibrations  and  Tones  produced  by  the  contact  of  Bodies  of  different  Temperatures 
—The  Trevelyan  Instrument— Rotation  of  hollow  Spheres  by  Electricity- 
Effect  of  Pressure  on  Fusing  Point— The  Fusing  Point  of  Bodies  which  con- 
tract on  solidifying  is  raised  by  Pressure— The  Fusing  Point  of  Bodies  which 
expand  on  solidifying  is  lowered  by  Pressure— Liquefaction  of  Ice  by  Pressure 
—Dissection  of  Ice  by  Calorific  Beam— Negative  Crystallisation,  Ice  reduced 
internally  to  Liquid  Flowers,  having  six  Petals  each— Central  Spot  a  Vacuum 
—Sound  heard  when  the  Spot  is  formed— Physical  Properties  of  Water  from 
•which  Air  has  been  removed— Its  Cohesion  enormously  augmented— It  can  be 


CONTENTS.  IX 

heated  far  above  its  boiling  Point— Its  Ebullition  becomes  Explosion— Applica- 
tion of  this  Property  to  explain  the  Sound  heard,  when  the  central  Spot  is 
formed  in  Ice— Possible  bearing  of  this  Property  of  Water  on  Boiler  explosions 
—The  Boiling  Point  of  Liquids— Resistance  to  Ebullition— Cohesion  of  Particles, 
adhesion  to  Vessel,  external  Pressure— Boiling  Points  on  various  Alpine  Sum- 
mits — The  law  of  Conservation  illustrated  in  the  Steam  Engine— The  Geysers 
of  Iceland — Description  of  the  Geysers  and  their  Phenomena — Bunsen's  Theory 
— Experimental  Illustration PAGB  114 

APPENDIX  TO   LECTURE   IV. 

Abstract  of  a  Lecture  on  the  Vibrations  and  Tones  produced  by  the  contact  of 
Bodies  having  different  Temperatures — Extracts  from  a  Paper  on  the  Physical 
Properties  of  Ice 144 

LECTURE  V., 

Application  of  the  Dynamical  Theory  to  the  Phenomena  of  Specific  and  Latent 
Heat — Definition  of  Energy — Potential  and  Djoiarnic  Energy,  illustrated  by 
the  Raising  and  Falling  of  a  Weight— Convertibility  of  Potential  into  Dynam- 
ic Energy  and  the  reverse— Constancy  of  the  Sum  of  both  Energies— A  ppli- 
-  cation  of  the  Ideas  of  Potential  and  Dynamic  Energy  to  Atoms  and  Molecules 
— Magnitude  of  Molecular  Forces — The  separation  of  a  Body's  Particles  by  Heat 
is  an  Act  the  same  in  kind  as  the  separation  of  a  Weight  from  the  Earth— 
Work  is  here  done  within  the  heated  Body — Interior  Work — The  Heat  com- 
municated to  a  Body  divides  itself  into  Potential  and  Dynamic  Energy— All 
Single  Atoms,  whatever  be  their  weight,  possess  the  same  amount  of  Dynamic 
Energy — Specific  Heat  or  Capacity  for  Heat  explained  by  Reference  to  Interior 
Work  and  to  Atomic  Number— Experimental  Illustrations  of  Specific  Heat- 
Table  of  Specific  Heats— Influence  of  high  Specific  Heat  of  Water  on  Climate 
— Heat  consumed  in  Change  of  Aggregation — Latent  Heat  of  Liquids  and  Va- 
pours— It  is  Heat  consumed  in  conferring  Potential  Energy  on  the  ultimate 
Particles— By  Condensation  and  Liquefaction  this  Potential  Energy  is  con- 
verted into  Heat — Mechanical  Value  of  the  Union  of  Oxygen  and  Hydrogen — 
Mechanical  Value  of  the  Change  from  Steam  to  liquid  Water — Mechanical 
Value  of  the  Change  from  liquid  Water  to  solid  Ice— Experimental  Illustra- 
tions— Consumption  and  Generation  of  Heat  by  Changes  of  Aggregation — 
Water  frozen  by  its  own  Evaporation— The  Cryophorus— Solid  Carbonic  Acid 
— The  Spheroidal  State  of  Liquids— In  this  State  the  Liquid  is  supported  on  a 
Bed  of  its  own  Vapour — Proofs  that  the  Spheroidal  Drop  is  not  in  Contact  with 
the  hot  Surface  underneath  it— Experimental  Illustrations  of  the  Spheroidal 
Condition — Possible  Bearing  of  these  Facts  on  the  Fiery  Ordeal,  and  on  Boiler 
Explosions — Freezing  of  Water  and  Mercury  in  red-hot  Vessels  .  .  152 

LECTURE  VI. 

Convection  in  Air — Larger  physical  Phenomena — Winds  caused  by  the  heating 
Action  of  the  Sun— The  upper  and  lower  Trade  Winds— Effect  of  the  Earth's 
Rotation  in  changing  the  apparent  Direction  of  Winds— The  Existence  of  tho 
upper  Current  proved  by  the  Discharge  of  Ashes  into  it  by  Volcanoes — Effects 
which  accompany  the  apparent  Motion  of  the  Sun  from  side  to  side  of  the 
Equator— Aqueous  Vapour— Tropical  Rains— Region  of  Calms— Europe,  for 
the  most  Part  in  tho  upper  Trade— Europe  the  Condenser  of  the  Western  At- 
1* 


X  CONTENTS. 

lantic— This  is  the  Cause  of  the  Mildness  of  European  Temperature— Rainfall 
in  Ireland — Effect  of  Mountain  Ranges  on  Rainfall— Convection  in  Liquids — 
Experimental  Illustrations— The  Gulf  Stream  :  its  influence  on  the  Climate 
of  Britain — Formation  of  Snow — The  Molecules  aggregate  to  form  Frozen 
Stars  with  Rays  sixty  degrees  apart— Figures  of  Snow  Crystals— Collection  of 
Snow  on  Mountains— The  Snow  Line— Squeezing  of  this  Snow  to  Ice— Forma- 
tion of  Glaciers— The  Motion  of  a  Glacier  resembles  that  of  a  River— Theories 
of  Glacier  Motion— The  Regelation  of  Ice— The  Moulding  of  Ice  by  Pressure- 
Ancient  Glaciers— Their  Traces  in  Switzerland,  England,  Ireland,  and  Wales 
— The  Cedars  of  Lebanon  grow  on  Glacier  Moraines — Theories  of  the  Glacial 
Epoch— Not  due  to  a  Diminution  of  Solar  Power,  or  to  the  passage  of  the  Solar 
System  through  cold  Regions  of  Space  ......  PAGE  185 

APPENDIX  TO  LECTURE  VI. 

Abstract  of  a  Lecture  on  the  Mer  de  Glaco 212 

LECTURE  VII. 

The  Conduction  of  Heat  a  Transmission  of  Molecular  Motion— Different  Bodies 
possess  different  Powers  of  Transmission — Good  Conductors  and  bad  Con- 
ductors—Experimental Illustrations — Experiments  of  Ingenhausz,  Despretz, 
Wiedemann,  and  Franz— Table  of  Conductivities— Parallelism  of  Conduction  of 
Heat  and  Conduction  of  Electricity — Good  Conductors  of  the  one  are  also  good 
Conductors  of  the  other,  and  vice  versa— Influence  of  Heat  on  Electric  Con- 
duction— The  Motion  of  Heat  interferes  with  the  Motion  of  Electricity— Con- 
duction of  Cold — Constancy  of  Temperature  of  Animal  Body — Capacity  to 
bear  high  Temperatures — Diversion  of  Heat  from  the  Purposes  of  Tempera- 
ture to  the  Performance  of  "Work— Influence  of  Molecular  Structure — Some 
Bodies  conduct  differently  in  different  Directions — Conduction  in  Crystals  and 
in  Wood — Feeble  Conductivity  of  Organic  Substances — This  secures  them 
from  sudden  Alternations  of  Temperature— Influence  of  Specific  Heat  on  the 
Speed  of  Conduction— Anomalous  Case  of  Bismuth  as  compared  with  Iron- 
Bismuth,  though  the  worst  Conductor,  apparently  transmits  Heat  most 
speedily — Action  of  Clothing — Rumford's  Experiments — Influence  of  mechani- 
cal Texture  on  Conduction— A  Powder  conducts  ill,  on  account  of  the  incessant 
Break  in  the  Continuity  of  the  Mass  along  which  the  Motion  of  Heat  is  trans- 
mitted— Non-conductivity  of  Gypsum — Effect  of  Boiler  encrustations — With- 
drawal of  Heat  from  Flames  by  good  Conductors— The  Motion  of  Flame, 
though  intense,  is  much  lowered  by  being  transferred  from  BO  light  a  Body  to 
a  heavy  one— Effect  of  Wire  Gauze— The  Safety  Lamp— Conduction  of  Liquids 
denied  by  Rumford,  but  proved  by  M.  Despretz— Conduction  of  Gases  denied 
by  Rumford,  but  affirmed  in  the  case  of  Hydrogen  by  Prof.  Magnus— Cooling 
Effect  of  Air  and  Hydrogen— Experiments  on  Gaseous  Conduction  doubt- 
ful   222 

LECTURE  VIII. 

Radiant  Heat — Cooling  a  loss  of  Motion — To  what  is  this  Motion  imparted? — Pre- 
liminary Experiments  on  Sound— Communication  of  Vibrations  through  the 
Air  to  Membranes  and  to  Flames — The  Vibrations  of  a  Body  propagated 
through  the  Air  and  striking  on  the  Drum  of  the  Ear  produce  Sound — Analo- 
gous Phenomena  of  Light— Theories  of  Emission  and  Undulation— Discussion* 


CONTENTS.  XI 

on  tho  Subject— Newton— Huyghens— Euler— Young— Frcsnel— Space  filled 
with  an  elastic  Medium  called  Ether— The  Motion  of  a  hot  or  of  a  luminous 
body  communicated  to  this  Ether  is  propagated  through  it  in  waves— In  this 
form  Heat  is  called  Radiant  Heat—ThQ  Thermo-electric  Pile  in  relation  to 
Radiant  Heat— Distribution  of  Heat  in  the  Electric  Spectrum  examined  ex- 
perimentally—Low Calorific  Power  of  blue  End  of  Spectrum— The  most 
luminous  Part  not  the  hottest  Part — Of  the  visible  rays  Red  is  the  hottest — 
The  maximum  Calorific  Action  is  beyond  the  Red,  and  is  due  to  Rays  which  are 
incompetent  to  excite  the  Sense  of  Vision— Extra  Violet  Rays— Physical  Cause 
of  Colour — The  Spectrum  is  to  the  Eye  what  the  Gamut  is  to  the  Ear — The 
Colour  of  Light  corresponds  to  the  Pitch  of  Sound — Number  of  Impulses  in- 
volved in  the  Perception  of  Light— Theory  of  Exchanges— Reflection  of  Radiant 
Heat  from  Plane  Surfaces— Angle  of  Incidence  equal  to  the  Anglo  of  Reflec- 
tion— Experimental  Proof— The  obscure  Rays  of  the  Electric  Lamp  pursue  the 
same  Track  as  the  luminous  ones — Angular  Velocity  of  reflected  Ray  twice 
that  of  rotating  Mirror— Experiments  with  radiant  Heat  of  Fire  and  of  obscure 
Bodies — Reflection  from  curved  Surfaces — Parabolic  Mirrors — Explosion  of 
Chlorine  and  Hydrogen  in  Focus  of  Mirror  by  Light— Explosion  of  Oxygen 
and  Hydrogen  by  Radiant  Heat — Reflection  of  Cold  .  .  .  PAGE  261 

APPENDIX  TO  LECTURE  VIII. 

On  the  Sounds  produced  by  the  Combustion  of  Gases  in  Tubes      .       .       .     288 

LECTURE  IX. 

The  Intensity  of  Radiant  Heat  diminishes  as  the  Square  of  the  Distance  from  tho 
radiant  Point  increases — Experimental  Proof— .Undulations  of  Sound  longitu- 
dinal, of  Light  transversal— The  ultimate  Particles  of  different  Bodies  possess 
different  Powers  of  Communicating  Motion  to  the  Ether — Experimental  Illus- 
trations of  good  and  bad  Radiators — Reciprocity  of  Radiation  and  Absorption 
—Protection  by  Gilding  against  Radiant  Heat— Transmission  of  Radiant  Heat 
through  Solids  and  Liquids— Diathermancy— Absorption  occurs  within  tho 
Body— Absorption  of  Light  by  Water— Radiant  Heat  passes  through  Diather- 
mic Bodies  without  heating  them — Athermic  Bodies  are  heated — Concentra- 
tion of  Beam  on  bulb  of  Air  Thermometer — Penetrative  Power  of  Sunbeams — 
—Sifting  of  Radiant  Heat— Ratio  of  Obscure  to  Luminous  Radiation  in  various 
Flames 301 

APPENDIX  TO  LECTURE  IX. 

On  some  physical  Properties  of  Ice 328 

LECTURE  X. 

Absorption  of  Heat  by  Gases  and  Vapours— First  Apparatus— Rockealt  Plates- 
Peculiarities  of  the  Galvanometer— The  higher  Degrees  of  greater  Value  than 
the  lower  ones— Improved  Apparatus— Principle  of  Compensation  permits  the 
use  of  a  powerful  Source  of  Heat  while  it  preserves  the  Needle  in  a  sensitive 
Position— Air,  Oxygen,  Hydrogen,  and  Nitrogen  are  practical  Vacua  to  Radiant 
Heat— Opacity  of  Olofiant  Gas  and  Sulphuric  Ether— Radiation  through  other 
Gases  and  Vapours — Great  difference  of  Absorbing  Power — Radiation  of  Heat 
by  Gases — The  atom  which  Absorbs  powerfully  Radiates  powerfully— Absorp- 
tion by  Gases  at  a  Tension  of  an  Atmosphere— Absorptions  at  smaller  Tensions 


Xll  CONTENTS. 

—Comparison  of  Elementary  and  Compound  Gases  and  Vapours— Radiation 
through  Lampblack,  &c PAGE  341 

APPENDIX  TO  LECTURE  X. 

Calibration  of  the  Galvanometer 3T6 

LECTURE  XL 

Action  of  Odorous  Substances  on  Radiant  Heat — List  of  Perfumes  examined— Ac- 
tion of  Ozone  on  Radiant  Heat— Influence  of  the  Size  of  the  Electrodes  on  the 
Quantity  of  Ozone  generated — Constitution  of  Ozone — Radiation  and  Absorp- 
tion of  Gases  and  Vapours  determined  without  any  Source  of  Heat  external  to 
the  gaseous  Body  itself— Dynamic  Radiation  and  Absorption— Varnishing  a 
metal  Surface  by  a  Gas — Varnishing  of  a  Gas  by  a  Vapour — Tenuity  of  Bora- 
cic  Ether  shown  in  Experiments  on  Dynamic  Radiation— Influence  of  Length 
of  radiating  Column — In  a  long  Tube,  a  Vapour  at  a  small  Tension  may  ex- 
ceed a  Gas  at  a  high  Tension,  while  in  a  short  Tube  the  Gas  exceeds  the 
Vapour— Radiation  through  Humid  Air— Action  of  the  Vapour  of  the  Atmo- 
sphere on  Terrestial  and  Solar  Radiation — Objections  answered— Applicability 
of  Rocksalt  Plates— Experiments  in  Tube  without  Plates— Experiments  with- 
out either  Tube  or  Plates— Examination  of  Air  from  various  localities— Influ- 
ence of  the  Results  on  the  Science  of  Meteorology— Application  to  tropical 
Rain  Torrents — To  the  Formation  of  Cumuli — To  the  Condensation  of  a  Moun- 
tainous Region— To  Radiation  Experiments  at  high  and  low  Elevations — To 
the  Cold  of  Central  Asia— To  the  Thermometric  Range  in  Australia— To  the 
Meteorology  of  Sahara — To  Leslie's  Experiments — To  "VVells's  Theory  of  Ice- 
formation  in  India— To  Melloni's  Theory  of  Serein  ,  379 

APPENDIX  TO  LECTUKE  XI. 

Extracts  from  a  Discourse  on  Eadiation  through  the  Earth's  Atmosphere — Thermo- 
metric range  in  Asia,  Africa,  and  Australia .  415 

LECTURE  XII. 

Examination  of  the  Diathermancy  of  Volatile  Liquids  and  their  Vapours — Apparatus 
for  this  purpose — Eocksalt  Cell — Platinum  Lamp — Experimental  arrangement  of 
Apparatus  for  determining  Absorption  of  Heat  by  Liquids — Table  of  the  Absorp- 
tion of  Heat  by  Liquids  at  various  thicknesses — Table  of  the  Absorption  of  Heat 
of  the  same  quality  by  the  Vapours  of  those  Liquids — Absorption  of  Heat  by  th« 
same  Vapours  when  the  quantities  of  Vapour  are  proportional  to  the  quantities 
of  Liquid — Comparative  View  of  the  Action  of  Liquids  and  their  Vapours  on 
Radiant  Heat — Predominance  of  Liquid  Water  as  an  Absorbent  fixes  the  predom- 
inance of  its  Vapour — Physical  cause  of  Transparency  and  Opacity — Transpar- 
ency defined  as  the  Discord,  and  Opacity  as  the  Accord,  between  the  Vibrating 
Periods  of  the  Source,  and  the  interposed  Substance — Influence  of  Temperature 
on  the  Transmission  of  Eadiant  Heat— Changes  of  Position  through  changes  of 
Temperature — Eadiation  from  various  Flames  through  Vapours — Periods  of  Vi 
bration  of  a  Hydrogen  Flame  proved  to  coincide  with  those  of  cool  Aqueous  Va 
pour— The  same  true  of  a  Carbonic  Oxide  Flame  and  cool  Carbonic  Acid— Physi- 
cal Analysis  of  the  Human  Breath— Eadiation  through  Liquids,  with  a  Hydrogen 


CONTENTS.  X1H 

Flame  and  a  Platinum  Spiral  in  a  Hydrogen  Flame  as  Sources  of  Heat— Exptona-" 
tion  of  certain  results  obtained  by  Melloni  and  Knoblauch         .        .        PAGE  423 

APPENDIX  TO  LECTURE  XII. 

On  Luminous  and  Obscure  Eadiation 456 

LECTURE  XIII. 

Dew :  a  clear  Sky  and  calm  but  deep  Atmosphere  necessary  for  its  copious  Forma- 
tion— Dewed  Substances  colder  than  undewed  ones — Dewed  Substances  better 
Eadiators  than  undewed  ones — Theory  of  Wells — Dew  is  the  Condensation  of  the 
Atmospheric  Vapour  on  Substances  which  have  been  chilled  by  Eadiation — Lunar 
Eadiation— Constitution  of  the  Sun — The  bright  Lines  in  the  Spectra  of  the  Met- 
als— An  incandescent  Vapour  absorbs  the  Eays  which  it  can  itself  emit — Kirch- 
hoff's  Generalisation — Fraunhofer's  Lines,  caused  by  the  Absorption  of  such  Eays 
by  the  luminous  Solar  Atmosphere  as  that  Atmosphere  itself  could  emit — Solar 
Chemistry — Emission  by  the  Sun — Herschel  and  Pouillet's  Experiments — Mayer's 
Meteoric  Theory— Eflect  of  the  Tides  on  the  Earth's  Eotation— Energies  of  the 
Solar  System — Ilelmholtz,  Herschel,  Thomson,  Waterston — Eelation  of  the  Sun 
to  Vegetable  and  Animal  Life— Form  of  Solar  Energy  in  Plants  and  Animals  470 

APPENDIX  TO  LECTURE  XIII. 

Extract  from  a  Lecture  on  the  Physical  Basis  of  Solar  Chemistry — Extract  from  a 
Paper  by  Dr.  Joule — Extracts  from  Dr.  Mayer's  Paper  on  Organic  Motion  and 
Nutrition  .  516 


HEAT 

CONSIDERED  AS 

A  MODE   OF  MOTION. 


LECTURE    I. 

[January  23,  1862.] 

INSTRUMENTS — GENERATION  OF  HEAT  BY  MECHANICAL  ACTION — 
CONSUMPTION  OP  HEAT  IN  WORK. 


APPENDIX :— NOTES    ON    THE    THERMO-ELECTRIC    PILE    AND    GALVANOMETER. 

THE  aspects  of  nature  provoke  in  man  the  spirit  of 
enquiry.  As  the  eye  is  made  for  seeing,  and  the 
ear  for  hearing,  so  the  human  mind  is  formed  for  under- 
standing the  phenomena  of  the  material  universe.  The 
natural  philosophy  of  our  day  results  from  the  irrepressi- 
ble exercise  of  this  endowment.  One  great  characteristic 
of  Natural  Science  is  its  growth ;  all  its  facts  are  fruitful, 
every  new  discovery  becoming  instantly  the  germ  of  fresh 
investigation.  But  no  nobler  example  of  this  growth 
could  be  adduced  than  the  expansion  and  development 
which  men's  thoughts  and  knowledge  have  undergone 
within  the  last  two-and-twenty  years,  with  reference  to  the 
subject  which  is  now  to  occupy  our  attention.  In  scien- 
tific manuals,  only  scanty  reference  has,  as  yet,  been 
made  to  the  modern  philosophy  of  Heat,  and  thus  the 
public  knowledge  regarding  it  remains  below  the  attain- 


14  LECTUKE   I. 

able  level.  But  the  reserve  is  natural,  for  the  subject  is 
still  an  entangled  one,  and,  in  entering  upon  it,  we  must  be 
prepared  to  encounter  difficulties.  In  the  whole  range  of 
Natural  Science,  however,  there  are  none  more  worthy  of 
being  overcome, — none  whose  subjugation  secures  a  greater 
reward  to  the  worker.  For  by  mastering  the  laws  and  re- 
lations of  Heat,  we  make  clear  to  our  minds  the  interde- 
pendence of  natural  forces  generally.  Let  us,  then,  com- 
mence our  labours  with  heart  and  hope ;  let  us  familiarise 
ourselves  with  the  latest  facts  and  conceptions  regarding 
this  all-pervading  agent,  and  seek  diligently  the  links  of 
law  which  underlie  the  facts  and  give  unity  to  their  most 
diverse  appearances.  If  we  succeed  here  we  shall  satisfy, 
to  an  extent  unknown  before,  that  love  of  order  and  of 
beauty  which,  I  am  persuaded,  is  implanted  in  the  mind 
of  every  person  here  present.  From  the  heights  at  which 
we  aim,  we  shall  have  nobler  glimpses  of  the  system  of 
Nature  than  could  possibly  be  obtained,  if  I,  while  acting 
as  your  guide  in  the  region  which  we  are  now  about  to  en- 
ter, were  to  confine  myself  to  its  lower  levels  and  already 
trodden  roads. 

It  is  my  first  duty  to  make  you  acquainted  with  some 
of  the  instruments  which  I  intend  to  employ  in  the  exami- 
nation of  this  question.  I  must  devise  some  means  of 
making  the  indications  of  heat  and  cold  visible  to  you  all, 
and  for  this  purpose  an  ordinary  thermometer  would  be 
useless.  You  could  not  see  its  action ;  and  I  am  anxious 
that  you  should  see,  with  your  own  eyes,  the  facts  on 
which  our  subsequent  philosophy  is  to  be  based.  I  wish  to 
give  you  the  material  on  which  an  independent  judgment 
may  be  founded ;  to  enable  you  to  reason  as  I  reason  if  you 
deem  me  right,  to  correct  me  if  I  go  astray,  and  to  censure 
me  if  you  find  me  dealing  unfairly  with  my  subject.  To 
secure  these  ends,  I  have  been  obliged  to  abandon  the  use 
of  a  common  thermometer,  and  to  resort  to  the  little  in- 


THE   THEKMO-ELECTEIC   PILE  AND   GALVANOMETER.     15 

strument  A  B  (fig.  1),  which  you  see  before  me  on  the  table, 
and  which  is  called  a  thermo-electric  pile.* 

By  means  of  this  instrument  I  cause  the  heat  which  it 
receives  to  generate  an  electric  current.  You  know,  or 
ought  to  knoAV,  that  such  a  current  has  the  power  of  de- 
flecting a  freely  suspended  magnetic  needle,  to  which  it 
flows  parallel.  Before  you  I  have  placed  such  a  needle 
m  n  (fig.  1),  surrounded  by  a  covered  copper  wire,  the  free 

Fig.  1. 


ends  of  which,  w  w  are  connected  with  the  thermo-electric 
pile.  The  needle  is  suspended  by  a  fibre,  s  s,  of  unspun 
silk,  and  protected  by  a  glass  shade,  G,  from  any  disturb- 
ance by  currents  of  air.  To  one  end  of  the  needle  I  have 
fixed  a  piece  of  red  paper,  and  to  the  other  end  a  piece  of 
blue.  All  of  you  see  these  pieces  of  paper,  and  when  the 
needle  moves,  its  motion  will  be  clearly  visible  to  the  most 
distant  person  in  this  room.f 

*  A  brief  description  of  the  thermo-electric  pile  is  given  in  the 
Appendix  to  this  Lecture. 

f  In  the  actual  arrangement  the  galvanometer  here  described  stood  on. 
a  stool  in  front  of  the  lecture  table,  the  wires  w  w,  being  sufficiently  long 


16  LECTURE   I. 

At  the  present  moment  the  needle  is  quite  at  rest,  and 
points  to  the  zero  mark  on  the  graduated  disc  underneath 
it.  This  shows  that  there  is  no  current  passing.  '  I  now 
breathe  for  an  instant  against  the  naked  face  A  of  the  pile 
— a  single  puff  of  breath  is  sufficient  for  my  purpose — 
observe  the  effect.  The  needle  starts  off  and  passes  through 
an  arc  of  90°.  It  would  go  further,  did  I  not  limit  its 
swing  by  fixing,  edgewise,  a  thin  plate  of  mica  at  90°. 
Take  notice  of  the  direction  of  the  deflection ;  the  red  end 
of  the  needle  moved  from  me  towards  you,  as  if  it  disliked 
me,  and  had  been  inspired  by  a  sudden  affection  for  you. 
This  action  of  the  needle  is  produced  by  the  small  amount 
of  warmth  communicated  by  my  breath  to  the  face  of  the 
pile,  and  no  ordinary  thermometer  could  give  so  large  and 
prompt  an  indication.  We  will  let  the  heat  thus  communi- 
cated waste  itself;  it  will  do  so  in  a  very  short  time,  and 
you  notice,  as  the  pile  cools,  that  the  needle  returns  to  its 
first  position.  Observe,  now,  the  effect  of  cold  on  the  face 
of  the  pile.  I  have  here  some  ice,  but  I  do  not  wish  to  wet 
my  instrument  by  touching  it  with  ice.  Instead  of  doing 
so,  I  will  cool  this  plate  of  metal  by  placing  it  on  the  ice ; 
then  wipe  the  chilled  metal,  and  touch  with  it  the  face  of 
the  pile.  You  see  the  effect ;  a  moment's  contact  suffices 
to  produce  a  prompt  and  energetic  deflection  of  the  needle. 
But  mark  the  direction  of  the  deflection.  When  the  pile 
was  warmed,  the  red  end  of  the  needle  moved  from  me 
towards  you ;  now  its  likings  are  reversed,  and  the  red  end 
moves  from  you  towards  me.  Thus  you  see  that  cold  and 
heat  cause  the  needle  to  move  in  opposite  directions.  The 
important  point  here  established  is,  that  from  the  direction 
in  which  the  needle  moves,  we  can,  with  certainty,  infer 
whether  cold  or  heat  has  been  communicated  to  the  pile  ; 

to  reach  from  the  table  to  the  stool ;  for  a  further  description  of  the  gal- 
vanometer, see  the  Appendix  to  this  Lecture. 


HEAT  OF  FRICTION.  IT 

and  the  energy  with  which  the  needle  moves — the  prompt- 
ness with  which  it  is  driven  aside  from  its  position  of  rest 
— gives  us  some  idea  of  the  comparative  quantity  of  heat 
or  cold  imparted  to  it  in  different  cases.  In  a  future  lecture 
I  shall  explain  how  we  may  express  the  relative  quantities 
of  heat  with  numerical  accuracy ;  but  for  the  present  a  gen- 
eral knowledge  of  the  action  of  our  instruments  will  be 
sufficient. 

My  desire  now  is  to  connect  heat  with  the  more  famil- 
iar forms  of  force,  and  I  will,  therefore,  in  the  first  place, 
try  to  furnish  you  with  a  store  of  facts  illustrative  of  .the 
generation  of  heat  by  mechanical  processes.  I  have  placed 
some  pieces  of  wood  in  the  next  room,  which  my  assistant 
will  now  hand  to  me.  Why  have  I  placed  them  there  ? 
Simply  that  I  may  perform  my  experiments  with  that  sin- 
cerity of  mind  and  act  which  science  demands  from  her 
cultivators.  I  know  that  the  temperature  of  that  room  is 
slightly  lower  than  the  temperature  of  this  one,  and  that 
hence  the  wood  which  is  now  before  me  must  be  slightly 
colder  than  the  face  of  the  pile  with  which  I  intend  to  test 
the  temperature  of  the  wood.  Let  us  prove  this.  I  place 
the  face  of  the  pile  against  this  piece  of  wood ;  the  red  end 
of  the  needle  moves  from  you  towards  me,  thus  showing 
that  the  contact  has  chilled  the  pile.  I  now  carefully  rub 
the  face  of  the  pile  along  the  surface  of  the  wood ;  I  say 
'  carefully,'  because  the  pile  is  a  brittle  instrument,  and 
rough  usage  would  destroy  it ; — mark  what  occurs.  The 
prompt  and  energetic  motion  of  the  needle  towards  you 
declares  that  the  face  of  the  pile  has  been  heated  by  this 
small  amount  of  friction.  The  needle,  you  observe,  goes 
quite  up  to  90°  on  the  side  opposite  to  that  towards  which 
it  moved  before  the  friction  was  applied. 

Now  these  experiments,  which  illustrate  the  develope- 
ment  of  heat  by  mechanical  means,  must  be  to  us  what  a 
boy's  school  exercises  are  to  him.  In  order  to  fix  them  on 


18  LECTUKE  I. 

our  minds,  and  obtain  due  mastery  over  them,  we  must  re- 
peat and  vary  them  in  many  ways.  In  this  task  I  ask  you 
to  accompany  me.  Here  is  a  flat  piece  of  brass  with  a  stem 
attached  to  it ;  I  take  the  stem  in  my  fingers,  preserving 
the  brass  from  all  contact  with  my  warm  hand,  by  envelop- 
ing the  stem  in  cold  flannel.  I  place  the  brass  in  contact 
with  the  face  of  my  pile  ;  the  needle  moves,  showing  that 
the  brass  is  cold.  I  now  rub  the  brass  against  the  surface 
of  this  cold  piece  of  wood,  and  lay  it  once  more  against 
my  pile.  I  withdraw  it  instantly,  for  it  is  so  hot  that  if  I 
allowed  it  to  remain  in  contact  with  the  instrument,  the 
current  generated  would  dash  my  needle  violently  against 
its  stops,  and  probably  derange  its  magnetism.  You  see 
the  strong  deflection  which  even  an  instant's  contact  can 
produce.  Indeed,  when  a  boy  at  school,  I  have  often  blis- 
tered my  hand  by  the  contact  of  a  brass  button,  which  I 
had  rubbed  energetically  against  a  form.  Here,  also,  is  a 
razor,  cooled  by  contact  with  ice  ;  and  here  is  a  hone,  with- 
out oil,  along  which  I  rub  my  cool  razor,  as  if  to  sharpen 
it.  I  now  place  the  razor  against  the  face  of  the  pile, 
and  you  see  that  the  steel,  which  a  minute  ago  was  cold, 
is  now  hot.  Similarly,  I  take  this  knife  and  knife-board, 
which  are  both  cold,  and  rub  the  knife  along  the  board.  I 
place  the  knife  against  the  pile,  and  you  observe  the  result ; 
a  powerful  deflection,  which  declares  the  knife  to  be  hot. 
I  pass  this  cold  saw  through  this  cold  piece  of  wood,  and 
place,  in  the  first  instance,  the  surface  of  the  wood  against 
which  the  saw  has  rubbed,  in  contact  with  the  pile.  The 
needle  instantly  moves  in  a  direction  which  shows  the 
wood  to  be  heated.  I  allow  the  needle  to  return  to  zero, 
and  now  apply  the  saw  to  the  pile.  It  also  is  hot.  These 
are  the  simplest  and  most  common-place  examples  of  the 
generation  of  heat  by  friction,  and  I  choose  them  for  this 
reason.  Mean  as  they  appear,  they  will  lead  us  by  degrees 


HEAT  OF  COMPRESSION  AND  PERCUSSION.  19 

into  the  secret  recesses  of  Nature,  and  lay  open  to  our 
view  the  policy  of  the  material  universe. 

Let  me  now  make  an  experiment  to  illustrate  the  de- 
velopement  of  heat  by  compression.  I  have  here  a  piece 
of  deal,  cooled  below  the  temperature  of  the  room,  and 
giving,  when  placed  in  contact  with  our  pile,  the  deflection 
which  indicates  cold.  I  place  this  wood  between  the 
plates  of  a  small  hydraulic  press,  and  squeeze  it  forcibly. 
The  plates  of  the  press  are  also,  you  will  observe,  cooler 
than  the  air  of  the  room.  After  compression,  I  bring  the 
wood  into  contact  with  the  pile  ;  see  the  effect.  The  gal- 
vanometer declares  that  heat  has  been  developed  by  the 
act  of  compression.  Precisely  the  same  occurs  when  I 
place  this  lead  bullet  between  the  plates  of  the  press  and 
squeeze  it  thus  to  flatness. 

And  now  for  the  effect  of  percussion.  I  have  here  a 
cold  lead  bullet,  which  I  place  upon  this  cold  anvil,  and 
strike  it  with  a  cold  sledge  hammer.  The  sledge  descends 
with  a  certain  mechanical  force,  and  its  motion  is  suddenly 
destroyed  by  the  bullet  and  anvil ;  apparently  the  force  of 
the  sledge  is  lost.  But  let  us  examine  the  lead ;  you  see  it 
is  heated,  and  could  we  gather  up  all  the  heat  generated  by 
the  shock  of  the  sledge,  and  apply  it  without  loss  mechan- 
ically, we  should  be  able,  by  means  of  it,  to  lift  this  ham- 
mer to  the  height  from  which  it  fell. 

I  have  here  arranged  another  experiment,  which  is  almost 
too  delicate  to  be  performed  by  the  coarse  apparatus  neces- 
sary in  a  lecture,  but  which  I  have  made  several  times  be- 
fore entering  this  room  to-day.  Into  this  small  basin  I 
pour  a  quantity  of  mercury  which  has  been  cooled  in  the 
next  room.  I  have  coated  one  of  the  faces  of  my  thermo-. 
electric  pile  with  varnish,  so  as  to  defend  it  from  the  mer- 
cury, which  would  otherwise  destroy  the  pile ;  and,  thus 
protected,  I  can,  as  you  observe,  plunge  the  pile  into  the 
liquid  metal.  The  deflection  of  the  needle  shows  you  that 


20 


LECTURE   I. 


Fig.  2. 


the  mercury  is  cold.  Here  are  two  glasses  A  and  B  (fig. 
2),  swathed  thickly  round  by  listing,  which  will  effectually 
prevent  the  warmth  of  my  hands  from  reaching  the  mer- 
cury. "Well,  I  pour  the  cold  mercury  from  the  one  glass 
into  the  other,  and  back.  It  falls  with  a  certain  mechani- 
cal force,  its  motion  is  destroyed,  but  heat  is  developed. 
The  amount  of  heat  generated  by  a  single  pouring  out  is 
extremely  small ;  I  could  tell  you  the  exact  amount,  but 
shall  defer  quantitative  considerations  till  our  next  lecture ; 
so  I  pour  the  mercury  from  glass  to  glass  ten  or  fifteen 
times.  Now  mark  the  result,  when  the  pile  is  plunged  into 

the  mercury.  The 
needle  moves,  and  its 
motion  declares  that 
the  mercury,  which 
at  the  beginning  of 
the  experiment  was 
cooler  than  the  pile, 
is  now  warmer  than 
the  pile.  We  here 
introduce  into  the 
lecture-room  an  effect 
which  occurs  in  na- 
ture at  the  base  of  ev- 
ery waterfall.  There 
are  friends  before  me 

who  have  stood  amid  the  foam  of  Niagara.  Had  they, 
when  there,  dipped  sufficiently  sensitive  thermometers  into 
the  water  at  the  top  and  bottom  of  the  cataract,  they  would 
have  found  the  latter  a  little  warmer  than  the  former.  The 
sailor's  tradition,  also,  is  theoretically  correct ;  the  sea  is 
rendered  warmer  through  the  agitation  produced  by  a 
storm,  the  mechanical  dash  of  its  billows  being  ultimately 
converted  into  heat. 

Whenever  friction  is  overcome,  heat  is  produced,  and 


ETC.,   FLlfrT   AOT)   STEEL.      21 

the  heat  produced  is  the  measure  of  the  force  expended  in 
overcoming  the  friction.  The  heat  is  simply  the  primitive 
force  in  another  form,  and  if  we  wish  to  avoid  this  conver- 
sion, we  must  abolish  the  friction.  We  usually  put  oil 
upon  the  surface  of  a  hone,  we  grease  a  saw,  and  are  care- 
ful to  lubricate  the  axles  of  our  railway  carriages.  What 
are  we  really  doing  in  these  cases  ?  Let  us  get  general 
notions  first ;  we  shall  come  to  particulars  afterwards.  It  is 
the  object  of  a  railway  engineer  to  urge  his  train  bodily 
from  one  place  to  another ;  say  from  London  to  Edinburgh, 
or  from  London  to  Oxford,  as  the  case  may  be ;  he  wishes 
to  apply  the  force  of  his  steam,  or  of  his  furnace,  which 
gives  tension  to  the  steam,  to  this  particular  purpose. 
It  is  not  his  interest  to  allow  any  portion  of  that  force  to 
be  converted  into  another  form  of  force  which  would  not 
further  the  attainment  of  his  object.  He  does  not  want 
his  axles  heated,  and  hence  he  avoids  as  much  as  possible 
expending  his  power  in  heating  them.  In  fact,  he  has  ob- 
tained his  force  from  heat,  and  it  is  not  his  object  to  recon- 
vert the  force  thus  obtained  into  its  primitive  form.  For, 
for  every  degree  of  temperature  generated  by  the  friction 
of  his  axles,  a  definite  amount  would  be  withdrawn  from 
the  urging  force  of  his  engine.  There  is  no  force  lost  ab- 
solutely. Could  we  gather  up  all  the  heat  generated  by 
the  friction,  and  could  we  apply  it  all  mechanically,  vre 
should,  by  it,  be  able  to  impart  to  the  train  the  precise 
amount  of  speed  which  it  had  lost  by  the  friction.  Thus 
every  one  of  those  railway  porters  whom  you  see  moving 
about  with  his  can  of  yellow  grease,  and  opening  the  little 
boxes  which  surround  the  carriage  axles,  is,  without  know- 
ing it,  illustrating  a  principle  which  forms  the  very  solder 
of  Nature.  In  so  doing,  he  is  unconsciously  affirming  both 
the  convertibility  and  the  indestructibility  of  force.  He  is 
practically  asserting  that  mechanical  energy  may  be  con- 
verted into  heat,  and  that,  when  so  converted,  it  cannot 


22  LECTURE  I. 

still  exist  as  mechanical  energy,  but  that,  for  every  degree 
of  heat  developed,  a  strict  and  proportional  equivalent  of 
the  locomotive  force  of  the  engine  disappears.  A  station  is 
approached,  say  at  the  rate  of  thirty  or  forty  miles  an  hour ; 
the  brake  is  applied,  and  smoke  and  sparks  issue  from  the 
wheel  on  which  it  presses.  The  train  is  brought  to  rest — 
How  ?  Simply  by  converting  the  entire  moving  force  which 
it  possessed,  at  the  moment  the  brake  was  applied,  into  heat. 

So,  also,  with  regard  to  the  greasing  of  a  saw  by  a  car- 
penter. He  applies  the  muscular  force  of  his  arm  with  the 
express  object  of  getting  through  the  wood.  He  wishes  to 
tear  the  wood  asunder,  to  overcome  its  mechanical  cohesion 
by  the  teeth  of  his  saw.  When  the  saw  moves  stiffly,  on 
account  of  the  friction  against  its  flat  surface,  the  same 
amount  of  force  may  produce  a  much  smaller  effect  than 
when  the  implement  moves  without  friction.  But  in  what 
sense  smaller  ?  Not  absolutely  so,  but  smaller  as  regards 
the  act  of  sawing.  The  force  not  expended  in  the  sawing 
is  not  lost ;  it  is  converted  into  heat,  and  I  gave  you  an 
example  of  this  a  few  minutes  ago.  Here  again,  if  we 
could  collect  the  heat  engendered  by  the  friction,  and  apply 
it  to  urge  the  saw,  we  should  make  good  the  precise  amount 
of  work  which  the  carpenter,  by  neglecting  the  lubrication 
of  his  implement,  had  simply  converted  into  another  form 
of  power. 

We  warm  our  hands  by  rubbing,  and  in  the  case  of 
frostbite  we  thus  restore  the  necessary  heat  to  the  injured 
parts.  Savages  have  the  art  of  producing  fire  by  the  skil- 
ful friction  of  well-chosen  pieces  of  wood.  It  is  easy  to 
char  wood  in  a  lathe  by  friction.  From  the  feet  of  the 
labourers  on  the  roads  of  Hampshire  sparks  issue  copiously 
on  a  dark  night,  the  collision  of  their  iron-shod  shoes 
against  the  flints  producing  the  effect.  In  the  common 
flint  and  steel  the  particles  of  the  metal  struck  off  are  so 
much  heated  by  the  collision  that  they  take  fire  and  burn  in 


WATEK  BOILED  BY  FRICTION.  25 

temperature  as  178°.  At  the  end  of  two  hours  and  twenty 
minutes  it  was  200°,  and  at  two  hours  and  thirty  minutes 
from  the  commencement  the  water  actually  boiled!  Rum- 
ford's  description  of  the  effect  of  this  experiment  on  those 
who  witnessed  it,  is  quite  delightful.  4  It  would  be  diffi- 
cult,' he  says,  4  to  describe  the  surprise  and  astonishment 
expressed  in  the  countenances  of  the  bystanders  on  seeing 
so  large  a  quantity  of  water  heated,  and  actually  made  to 
boil,  without  any  fire.  Though  there  was  nothing  that 
could  be  considered  very  surprising  in  this  matter,  yet  I 
acknowledge  fairly  that  it  afforded  me  a  degree  of  childish 
pleasure  which,  were  I  ambitious  of  the  reputation  of  a 
grave  philosopher,  I  ought  most  certainly  rather  to  hide 
than  to  discover.'*  I  am  sure  that  both  you  and  I  can  dis- 
pense with  the  application  of  any  philosophy  which  would 
stifle  such  emotion  as  Rumford  here  avowed.  In  connec- 
tion with  this  striking  experiment,  Mr.  Joulef  has  estimated 
the  amount  of  mechanical  force  expended  in  producing  the 
heat,  and  obtained  a  result  which  '  is  not  very  widely  differ- 
ent '  from  that  which  greater  knowledge  and  more  refined 
experiments  enabled  Mr.  Joule  himself  to  obtain,  as  regards 
the  numerical  equivalence  of  heat  and  work. 

It  would  be  absurd  on  my  part  to  attempt  here  a  repe- 
tition of  the  experiment  of  Count  Rumford  with  all  its 
conditions.  I  cannot  devote  two  hours  and  a  half  to  a  sin- 
gle experiment,  but  I  hope  to  be  able  to  show  you  substan- 
tially the  same  effect  in  two  minutes  and  a  half.  I  have 
here  a  brass  tube,  four  inches  long,  and  three  quarters  of 
an  inch  in  interior  diameter.  It  is  stopped  at  the  bottom, 
and  I  thus  screw  it  on  to  a  whirling  table,  by  means  of 
which  I  can  cause  the  upright  tube  to  rotate  very  rapidly. 
I  have  here  two  pieces  of  oak  wood,  united  by  a  hinge,  and 


2 


*  Rumford's  Essays,  vol.  ii.  p.  484. 

f  Philosophical  Transactions,  vol.  cxl.  p.  62. 


26  LECTURE   I. 

in  which  are  two  semicircular  grooves,  which  are  intended 
to  embrace  the  brass  tube.  Thus  the  pieces  of  wood  form 
a  kind  of  tongs,  T  (fig.  3),  by  gently  squeezing  which  I  can 
produce  friction  between  the  wood  and  the  brass  tube, 
when  the  latter  rotates.  I  almost  fill  the  tube  with  cold 


Fig.  3. 


water,  and  stop  it  with  a  cork,  to  prevent  the  splashing  out 
of  the  liquid,  and  now  I  put  the  machine  in  motion.  As 
the  action  continues,  the  temperature  of  the  water  rises, 
and  though  the  two  minutes  and  a  half  have  not  yet  elapsed, 
those  near  the  apparatus  will  see  steam  escaping  from  the 
cork.  Three  or  four  times  to-day  I  have  projected  the  cork 
by  the  force  of  the  steam  to  a  height  of  twenty  feet  in  the 
air.  There  it  goes  again,  and  the  steam  follows  it,  pro- 
ducing by  its  precipitation  this  small  cloud  in  the  at- 
mosphere. 

In  all  the  cases  hitherto  introduced  to  your  notice,  heat 
has  been  generated  by  the  expenditure  of  mechanical  force. 
Our  experiments  have  gone  to  show  that  where  mechanical 
force  is  expended  heat  is  produced,  and  I  wish  now  to 
bring  before  you  the  converse  experiment,  that  is,  the  con- 
sumption of  heat  in  mechanical  work.  And  should  you  at 
present  find  it  difficult  to  form  distinct  conceptions  as  to 
the  bearing  of  these  experiments,  I  exhort  you  to  be  pa- 
tient. We  are  engaged  on  a  difficult  and  entangled  sub- 


COLD   OF  DILATATION.  If  "£  27 

ject,  which,  I  hope,  we  shall  disentangle  aa  we  go  along. 
I  have  here  a  strong  vessel,  filled,  at  the  present  moment, 
with  compressed  air.  It  has  been  now  compressed  for  some 
hours,  so  that  the  temperature  of  the  air  within  the  vessel 
is  the  same  as  that  of  the  air  of  the  room  without  it.  At 
the  present  moment,  then,  this  inner  air  is  pressing  against 
the  sides  of  the  vessel,  and  if  I  open  this  cock  a  portion  of 
the  air  will  rush  violently  out  of  the  vessel.  The  word 
4  rush,'  however,  but  vaguely  expresses  the  true  state  of 
things ;  the  air  which  rushes  out  is  driven  out  by  the  air 
behind  it ;  this  latter  accomplishes  the  work  of  urging  for- 
ward the  stream  of  air.  And  what  will  be  the  condition 
of  the  working  air  during  this  process  ?  It  will  be  chilled. 
It  performs  mechanical  wrork,  and  the  only  agent  which  it 
can  call  upon  to  perform  it  is  the  heat  which  it  possesses, 
and  to  which  the  elastic  force  with  which  it  presses  against 
the  sides  of  the  vessel,  is  entirely  due.  A  portion  of  this 
heat  will  be  consumed  and  the  air  will  be  chilled.  Observe 
the  experiment  which  I  am  about  to  make.  I  will  turn 
the  cock  c,  and  allow  the  current  of  air  from  the  vessel  v 
(fig.  4),  to  strike  against  the  face  of  the  pile  P.  See  how 

Fig.  4. 


28 


LECTTTEE  I. 


the  magnetic  needle  responds  to  the  act ;  its  red  end  is 
driven  towards  me,  thus  declaring  that  the  pile  has  been 
chilled  by  the  current  of  air. 

The  effect  is  different  when  a  current  of  air  is  urged 
from  the  nozzle  of  a  common  bellows  against  the  thermo- 
electric pile.  In  the  last  experiment  the  mechanical  work 
of  urging  the  air  forward  was  performed  by  the  air  itself, 
and  a  portion  of  its  heat  was  consumed  in  the  effort.  In 
the  case  of  the  bellows,  it  is  my  muscles  which  perform 
the  work.  I  raise  the  upper  board  of  the  bellows  and  the 
air  rushes  in ;  I  press  the  boards  with  a  certain  force,  and 
the  air  rushes  out.  The  expelled  air  strikes  the  face  of  the 
pile,  has  its  motion  stopped,  and  an  amount  of  heat  equiva- 
lent to  the  destruction  of  this  motion  is  instantly  generated. 
Thus  you  observe  that  when  I  urge  with  the  bellows  B 


Fig.  5. 


(fig.  5),  a  current  of  air  against  the  pile,  the  red  end  of  the 
needle  moves  towards  you,  thereby  showing  that  the  face 
of  the  pile  has  been,  in  this  instance,  warmed  by  the  air.  I 
have  here  a  bottle  of  soda  water  ;  at  present  the  bottle  is 
slightly  warmer  than  the  pile,  as  you  see  by  the  deflection 
it  produces  ;  I  cut  the  strings  which  holds  the  cork,  and  it 


COLD  OF  DILATATION.  29 

is  it  driven  out  by  the  elastic  force  of  the  carbonic  acid 
gas ;  the  gas  performs  work,  in  so  doing  consumes  heat, 
and  now  the  deflection  it  produces  is  that  of  cold.  The 
truest  romance  is  to  be  found  in  the  details  of  daily  life, 
and  here,  in  operations  with  which  every  child  is  familiar, 
we  shall  gradually  discern  the  illustration  of  principles 
from  which  all  material  phenomena  flow. 


APPENDIX   TO    LECTURE    I. 


Fig.  6. 


NOTE  ON  THE  CONSTEUCTION  OF  THE  THEEMO-ELECTEIC  PILE. 

LET  A  B  (fig.  6)  be  a  bar  of  antimony,  and  B  c  a  bar  of  bis- 
muth, and  let  both  bars  be  soldered  together  at  B.  Let  the  free 
ends  A  and  c  be  united  by  a  piece  of  wire,  ADC. 
On  warming  the  place  of  junction,  B,  an  electric 
current  is  generated,  the  direction  of  which  is 
from  bismuth  to  antimony  (B  to  A,  or  against 
the  alphabet),  across  the  junction,  and  from  an- 
timony to  bismuth  (A  to  B,  or  with  the  alpha- 
bet), through  the  connecting  wire,  ADC.  The 
arrow  indicates  the  direction  of  the  current. 

If  the  junction  B  be  chilled,  a  current  is  gene- 
rated opposed  in  direction  to  the  former.  The 
figure  represents  what  is  called  a  thermo-electric 
pair  or  couple. 

By  the  union  of  several  thermo-electric  pairs, 
a  more  powerful  current  can  be  generated  than 
would  be  obtained  from  a  single  pair.  Fig.  7, 
for  example,  represents  such  an  arrangement,  in  which  the  shaded 
bars  are  supposed  to  be  all  of  bismuth,  and  the  unshaded  ones  of 
antimony ;  on  warming  all  the  junctions,  B,  B,  &c.,  a  current  is 
generated  in  each,  and  the  sum  of  these  currents,  all  of  which  flow 
in  the  same  direction,  will  produce  a  stronger  resultant  current 
than  that  obtained  from  a  single  pair. 

The  V  formed  by  each  pair  need  not  be  so  wide  as  it  is  shown 
in  fig.  7 ;  it  may  be  contracted  without  prejudice  to  the  couple. 
And  if  it  is  desired  to  pack  several  pairs  into  a  small  compass, 


THEKMO-ELECTEICITY. 


31 


each  separate  couple  may  be  arranged  as  in  fig.  8,  where  the  black 
lines  represent  small  bismuth  bars,  and  the  white  ones  small  bars 
of  antimony.  They  are  soldered  together  at  the  ends,  and  through- 
out the  length  are  usually  separated  by  strips  of  paper  merely.  A 


collection  of  pairs  thus  compactly  set  together  constitutes  a  ther- 
mo-electric pile,  a  drawing  of  which  is  given  in  fig.  9. 

The  current  produced  by  heat  being  always  from  bismuth  to 
antimony  across  the  heated  junction,  a  moment's  inspection  of 
fig.  7  will  show  that  when  any  one  of  the  junctions  A,  A,  is  heated, 
a  current  is  generated,  opposed  in  direction  to  that  generated 
when  the  heat  is  applied  to  the  junctions  B,  B.  Hence,  in  the  case 
of  the  thermo-electric  pile,  the  effect  of  heat  falling  upon  its  two 


Fig.  8. 


Fig.  9. 


opposite  faces  is  to  produce  currents  in  opposite  directions.  If 
the  temperature  of  the  two  faces  be  alike,  they  neutralize  each 
other,  no  matter  how  high  they  may  be  heated  absolutely,  but  if 
one  of  them  be  warmer  than  the  other,  a  current  is  produced.  The 
current  is  thus  due  to  a  difference  of  temperature  between  the  two 
faces  of  the  pile,  and  within  certain  limits  the  strength  of  the  cur- 
rent is  exactly  proportioned  to  this  difference. 


32  APPENDIX  TO  LECTUKE  I. 

From  the  junction  of  almost  any  other  two  metals,  thermo- 
electric currents  may  be  obtained,  but  they  are  most  copiously 
generated  by  the  union  of  bismuth  and  antimony.* 


NOTE  ON  THE  CONSTRUCTION  OF  THE  GALVANOMETER. 

The  existence  and  direction  of  an  electric  current  are  shown 
by  its  action  upon  a  freely  suspended  magnetic  needle. 

But  such  a  needle  is  held  in  the  magnetic  meridian  by  the 
magnetic  force  of  the  earth.  Hence,  to  move  a  single  needle,  the 
current  must  overcome  the  magnetic  force  of  the  earth. 

Very  feeble  currents  are  incompetent  to  do  this  in  a  sufficiently 
sensible  degree.  The  following  two  expedients  are,  therefore, 
combined  to  render  sensible  the  action  of  such  feeble  currents : — 

The  wire  through  which  the  current  flows  is  coiled  so  as  to 
surround  the  needle  several  times  ;  the  needle  must  swing  freely 
within  the  coil.  The  action  of  the  single  current  is  thus  mul- 
tiplied. 

The  second  device  is  to  neutralize  the  directive  force  of  the 
earth,  without  prejudice  to  the  magnetism  of  the  needle.  This  is 
accomplished  by  using  two  nee- 
dles instead  of  one,  attaching 
them  to  a  common  vertical  stem, 
and  bringing  their  opposite  poles 
over  each  other,  the  north  end  of 
the  one  needle,  and  the  south  end  £ . 


of  the  other,  being  thus  turned  in 
the  same  direction.    The  double 

needle  is  represented  in  fig.  10.          2 

It  must  be  so  arranged  that 
one  of  the  needles  shall  be  within  the  coil  through  which  the  cur- 

*  The  discovery  of  thermo-electricity  is  due  to  Thomas  Seebeck,  Pro- 
fessor in  the  University  of  Berlin.  Nobili  constructed  the  first  thermo- 
electric pile ;  but  in  Melloni's  hands  it  became  an  instrument  so  important 
as  to  supersede  all  others  in  researches  on  radiant  heat.  To  this  purpose 
it  will  be  applied  in  future  lectures. 


THE  ASTATIC  NEEDLE. 


33 


rent  flows,  while  the  other  needle  swings  freely  above  the  coil,  the 
vertical  connecting  piece  passing  through  an  appropriate  slit  in 
the  coil.  Were  both  the  needles  within,  the  same  current  would 
urge  them  in  opposite  directions,  and  thus  one  needle  would  neu- 
tralize the  other.  But  when  one  is  within  and  the  other  without, 
the  current  urges  both  needles  in  the  same  direction. 

The  way  to  prepare  such  a  pair  of  needles  is  this.  Magnetize 
both  of  them  to  saturation ;  then  suspend  them  in  a  vessel,  or  un- 
der a  shade,  so  as  to  protect  them  from  air-currents.  The  system 
will  probably  set  in  the  magnetic  meridian,  one  needle  being  in 
almost  all  cases  stronger  than  the  other;  weaken  the  stronger 
needle  carefully  by  the  touch  of  a  second  smaller  magnet.  When 
the  needles  are  precisely  equal  in  strength,  they  will  set  at  right 
angles  to  the  magnetic  meridian. 

It  might  be  supposed  that  when  the  needles  are  equal  in 
strength,  the  directive  force  of  the  earth  would  be  completely  an- 
nulled, that  the  double  needle  would  be  perfectly  astatic,  and  per- 

Fig.  11. 


fectly  neutral  as  regards  direction ;  obeying  simply  the  torsion  of 
its  suspending  fibre.  This  would  be  the  case  if  the  magnetic  axes 
of  both  needles  could  be  caused  to  lie  with  mathematical  accuracy 
in  the  same  vertical  plane.  In  practice,  this  is  next  to  impos- 
2* 


34:  APPENDIX  TO   LECTURE   I. 

sible ;  tlie  axes  always  cross  each  other.  Let  n  s,  n'  s'  (fig.  11) 
represent  the  axes  of  two  needles  thus  crossing,  the  magnetic 
meridian  being  parallel  to  M  E  ;  let  th°.  pole  n  be  drawn  by  the 
earth's  attractive  force  in  the  direction  n  m ;  the  pole  s'  being 
urged  by  the  repulsion  of  the  earth  in  a  precisely  opposite  direc- 
'  tion.  When  the  poles  n  and  s'  are  of  exactly  equal  strength,  it  is 
manifest  that  the  force  acting  on  the  pole  s',  in  the  case  here  sup- 
posed, would  have  the  advantage  as  regards  leverage,  and  would 
therefore  overcome  the  force  acting  on  n.  The  crossed  needles 
would  therefore  turn  away  still  further  from  the  magnetic  meri- 
dian, and  a  little  reflection  will  show  that  they  cannot  come  to  rest 
until  the  line  which  bisects  the  angle  enclosed  by  the  needles  is  at 
right  angles  to  the  magnetic  meridian. 

This  is  the  test  of  perfect  equality  as  regards  thv,  magnetism 
of  the  needles  ;  but  in  bringing  the  needles  to  this  state  of  perfec- 
tion, we  have  often  to  pass  through  various  stages  of  obliquity  to 
the  magnetic  meridian.  In  these  cases  the  superior  strength  of 
one  needle  is  compensated  by  an  advantage,  as  regards  leverage, 
possessed  by  the  other.  By  a  happy  accident  a  touch  is  some- 
times sufficient  to  make  the  needles  perfectly  equal ;  but  many 
hours  are  often  expended  in  securing  this  result.  It  is  only,  of 
course,  in  very  delicate  experiments  that  this  perfect  equality  is 
needed  ;  but  in  such  experiments  it  is  essential. 

Another  grave  difficulty  has  beset  experimenters,  even  after  the 
perfect  magnetization  of  their  needles  has  been  accomplished. 
Such  needles  are  sensitive  to  the  slightest  magnetic  action,  and 
the  covered  copper  wire,  of  which  the  galvanometer  coils  are 
formed,  usually  contains  a  trace  of  iron  sufficient  to  deflect  the 
prepared  needle  from  its  true  position.  I  have  had  coils  in  which 
this  deflection  amounted  to  30  degrees ;  and  in  the  splendid  in- 
struments used  by  Professor  Du  Bois  Raymond,  in  his  researches 
on  animal  electricity,  the  deflection  by  the  coil  is  sometimes  even 
greater  than  this.  Melloni  encountered  this  difficulty,  and 
proposed  that  the  wires  should  be  drawn  through  agate  holes, 
thus  avoiding  all  contact  with  iron  or  steel.  The  disturb- 
ance has  always  been  ascribed  to  a  trace  of  iron  contained 
in  the  copper  wire.  Pure  silver  has  also  been  proposed  instead 
of  copper. 


THE   ASTATIC   NEEDLE.  35 

To  pursue  his  beautiful  thermo-electric  researches  in  a  satisfac- 
tory manner,  Professor  Magnus,  of  Berlin,  obtained  pure  copper, 
by  a  most  laborious  electrolytic  process,  and  after  the  metal  had 
been  obtained,  it  required  to  be  melted  eight  times  in  succession 
before  it  could  be  drawn  inio  wire.  In  fact,  the  impurity  of  the 
coil  entirely  vitiated  the  accuracy  of  the  instrument,  and  almost 
any  amount  of  labour  would  be  well  expended  in  removing  this 
great  defect. 

My  own  experience  of  this  subject  is  instructive.  I  had  a  beau- 
tiful instrument  constructed  a  few  years  ago  by  Sauerwald,  of  Ber- 
lin, the  coil  of  which,  when  no  current  flowed  through  it,  deflected 
my  double  needle  full  30  degrees  from  the  zero  line.  It  was  im- 
possible to  attain  quantitative  accuracy  with  this  instrument. 

I  had  the  wire  removed  by  Mr.  Becker,  and  English  wire  used 
in  its  stead  ;  the  deflection  fell  to  3  degrees. 

This  was  a  great  improvement,  but  not  sufficient  for  rny  pur- 
pose. I  commenced  to  make  inquiries  about  the  possibility  of 
obtaining  pure  copper,  but  the  result  was  very  discouraging, 
when,  almost  despairing,  the  following  thought  occurred  to  me : 
The  action  of  the  coil  must  be  due  to  the  admixture  of  iron  with 
the  copper,  for  pure  copper  is  diamagnetic,  it  is  feebly  repelled  by 
a  strong  magnet.  The  magnet  therefore  occurred  to  me  as  a 
means  of  instant  analysis ;  I  could  tell  t>y  it,  in  a  moment,  whether 
any  wire  was  free  from  the  magnetic  metal  or  not. 

The  wire  of  M.  Sauerwald's  coil  was  strongly  attracted  by  the 
magnet.  The  wire  of  Mr.  Becker's  coil  was  also  attracted,  though 
in  a  much  feebler  degree. 

Both  wires  had  been  covered  by  green  silk ;  I  removed  this, 
but  the  Berlin  wire  was  still  attracted  ;  the  English  wire,  on  the 
contrary,  when  presented  naked  to  the  magnet  was  feebly 
repelled;  it  was  truly  diamagnetic,  and  contained  no  sensible 
trace  of  iron.  Thus  the  whole  annoyance  was  fixed  upon  the 
green  silk ;  some  iron  compound  had  been  used  in  the  dyeing 
of  it,  and  to  this  the  deviation  of  the  needle  from  zero  was  mani- 
festly due. 

I  had  the  green  coating  removed  and  the  wire  overspun  with 
white  silk,  clean  hands  being  used  in  the  process.  A  perfect  gal- 
vanometer is  the  result ;  the  needle,  when  released  from  the  action 


36  APPENDIX  TO   LECTUKE  I. 

of  the  current,  returns  accurately  to  zero,  and  is  perfectly  free  from 
all  magnetic  action  on  the  part  of  the  coil.  In  fact,  while  we 
have  been  devising  agate  plates  and  other  learned  methods  to  get 
rid  of  the  nuisance  of  a  magnetic  coil,  the  means  of  doing  so  are 
at  hand.  Let  the  copper  wire  be  selected  by  the  magnet,  and  no 
difficulty  will  be  experienced  in  obtaining  specimens  magnetically 
pure. 


LECTURE    II. 

[January  30,  1862.] 


THE   NATURE    OP   HEAT — THE    MATERIAL   THEORY — THE    DYNAMICAL  THEORY 

— THERMAL     EFFECTS     OF     AIR     IN     MOTION GENERATION     OF     HEAT     BY 

ROTATION    BETWEEN    THE    POLES    OF    A    MAGNET EXPERIMENTS    OF    RUM- 
FORD,     DAVY,     AND     JOULE — THE     MECHANICAL     EQUIVALENT     OF     HEAT 

HEAT    GENERATED    BY    PROJECTILES HEAT  WHICH  WOULD   BE    GENERATED 

BY    STOPPING    THE     EARTH'S     MOTION — METEORIC    THEORY    OF    THE    SUN'S 
HEAT — FLAME   IN   ITS   RELATION   TO   THE   DYNAMICAL   THEORY. 


APPENDIX: — EXTRACTS  FROM  BACON  AND  RUMFORD. 

IN  our  last  lecture  the  developement  of  heat  by  mechan- 
ical action  was  illustrated  by  a  series  of  experiments, 
which  showed  that  heat  was  easily  produced  by  friction, 
by  compression,  and  by  percussion.  But  facts  alone  can 
not  satisfy  the  human  mind  ;  we  desire  to  know  the  inner 
and  invisible  cause  of  the  fact ;  we  search  after  the  prin- 
ciple by  the  operation  of  which  the  phenomena  are  pro- 
duced. Why  should  heat  be  generated  by  mechanical  ac- 
tion, and  what  is  the  real  nature  of  the  agent  thus  gene- 
rated ?  Two  rival  theories  have  been  offered  in  answer  to 
these  questions.  Till  very  lately,  however,  one  of  these — 
the,  material  theory — had  the  greater  number  of  adherents, 
being  opposed  by  only  a  few  eminent  men.  Within  cer- 
tain limits  this  theory  involved  conceptions  of  a  very  sun- 
pie  kind,  and  this  simplicity  secured  its  general  acceptance. 
The  material  theory  supposes  heat  to  be  a  kind  of  matter 
— a  subtle  fluid — stored  up  in  the  inter-atomic  spaces  of 


38  LECTURE  H. 

bodies.  The  laborious  Gmelin,  for  example,  in  his  Hand- 
book of  Chemistry,  defines  heat  to  be  4  that  substance 
whose  entrance  into  our  bodies  causes  the  sensation  of 
warmth,  and  its  egress  the  sensation  of  cold.'*  He  also 
speaks  of  heat  combining  with  bodies  as  one  ponderable 
substance  does  with  another;  and  many  other  eminent 
chemists  treat  the  subject  from  the  same  point  of  view. 

The  developement  of  heat  by  mechanical  means,  inas- 
much as  its  generation  seemed  unlimited,  was  a  great  diffi- 
culty with  the  materialists ;  but  they  were  acquainted  with 
the  fact  (which  I  shall  amply  elucidate  in  a  future  lecture) 
that  different  bodies  possessed  different  powers  of  holding 
heat,  if  I  may  use  such  a  term.  Take,  for  example,  the 
two  liquids  -water  and  mercury,  and  warm  up  a  pound  of 
each  of  them,  say  from  fifty  degrees  to  sixty.  The  abso- 
lute quantity  of  heat  required  by  the  water  to  raise  its 
temperature  10°  is  fully  thirty  times  the  quantity  required 
by  the  mercury.  Technically  speaking,  the  water  is  said 
to  have  a  greater  capacity  for  heat  than  the  merctfry  has, 
and  this  term  '  capacity '  is  sufficient  to  suggest  the  views 
of  those  who  invented  it.  The  water  was  supposed  to 
possess  the  power  of  storing  up  the  caloric  or  matter  of 
heat ;  of  hiding  it,  in  fact,  to  such  an  extent  that  it  required 
thirty  measures  of  this  caloric  to  produce  the  same  sensible 
effect  on  it,  that  one  measure  would  produce  upon  mercury. 

All  substances  possess,  in  a  greater  or  less  degree,  this 
apparent  power  of  storing  up  heat.  Lead,  for  example, 
possesses  it ;  and  the  experiment  with  the  lead  bullet,  in 
which  you  saw  heat  generated  by  compression,  was  explain- 
ed by  those  who  held  the  material  theory  in  the  follow- 
ing way.  The  uncompressed  lead,  they  said,  has  a  higher 
capacity  for  heat  than  the  compressed  substance ;  the  size 
of  its  atomic  storehouse  is  diminished  by  compression,  and 

*  English  Translation,  vol.  i.  p.  22. 


MATERIAL   AND  DYNAMICAL   THEORIES   OF   HEAT.        39 

hence,  when  the  lead  is  squeezed,  a  portion  of  that  heat 
which,  previous  to  compression,  was  hidden,  must  make  its 
appearance,  for  the  qompressed  substance  can  no  longer 
hold  it  all.  In  some  similar  way  the  experiments  on  fric- 
tion and  percussion  were  accounted  for.  The  idea  of  call- 
ing new  heat  into  existence  was  rejected  by  the  believers 
in  the  material  theory.  According  to  their  views,  the 
quantity  of  heat  in  the  universe  is  as  constant  as  the 
quantity  of  ordinary  matter,  and  the  utmost  we  can  do  by 
mechanical  and  chemical  means,  is  to  store  up  this  heat  or 
to  drive  it  from  its  lurking  place  into  open  light  of  day. 

The  dynamical  theory,  or,  as  it  is  sometimes  called,  the 
mechanical  theory  of  heat,  discards  the  idea  of  materiality 
as  applied  to  Ireat.  The  supporters  of  this  theory  do  not 
believe  heat  to  be  matter,  but  an  accident  or  condition  of 
matter ;  namely,  a  motion  of  its  ultimate  particles.  From 
the  direct  contemplation  of  some  of  the  phenomena  of 
heat,  a  profound  mind  is  led  almost  instinctively  to  con- 
clude that  heat  is  a  kind  of  motion.  Bacon  held  a  view  of 
this  kind,*  and  Locke  stated  a  similar  view  with  singular 
felicity.  '  Heat '  he  says,  '  is  a  very  brisk  agitation  of  the 
insensible  parts  of  the  object,  which  produce  in  us  that 
sensation  from  whence  we  denominate  the  object  hot ;  so 
what  in  our  sensation  is  heat,  in  the  object  is  nothing  but 
motion?  In  our  last  lecture  I  referred  to  the  experiments 
of  Count  Rumford  f  on  the  boring  of  cannon  ;  he  showed 
that  the  hot  chips  cut  from  his  cannon  did  not  change  their 
capacity  for  heat ;  he  collected  the  scales  and  powder  pro- 
duced by 'the  abrasion  of  his  metal,  and  holding  them  up 

*  See  Appendix  to  this  Lecture. 

\  I  have  particular  pleasure  in  directing  the  reader's  attention  to  an 
abstract  of  Count  Rumford's  memoir  on  the  Generation  of  Heat  by  Fric- 
tion, contained  in  the  Appendix  to  this  lecture.  Rumford,  in  this  me- 
moir, annihilates  the  material  theory  of  heat.  Nothing  more  powerful  on 
the  subject  has  since  been  written. 


40  LECTURE   H. 

before  his  opponents,  demanded  whether  they  believed 
that  the  vast  amount  of  heat  which  he  had  generated  had 
been  all  squeezed  out  of  that  modicum  of  crushed  metal  ? 
1  You  have  not,'  he  might  have  added,  '  given  yourselves 
the  trouble  to  enquire  whether  any  change  whatever  has 
occurred  in  the  capacity  of  the  metal  for  heat  by  the  act 
of  friction.  You  are  quick  in  inventing  reasons  to  save 
your  theory  from  destruction,  but  slow  to  enquire  whether 
these  reasons  are  not  merely  the  finespun  fancies  of  your 
own  brains.'  Theories  are  indispensable,  but  they  some- 
times act  like  drugs  upon  the  mind.  Men  grow  fond  of 
them  as  they  do  of  dram-drinking,  and  often  feel  discon- 
tented and  irascible  when  the  stimulant  to  the  imagination 
is  taken  away. 

At  this  point  an  experiment  of  Davy  comes  forth  in  its 
true  significance.*  Ice  is  solid  water,  and  the  solid  has 
only  one  half  the  capacity  for  heat  that  liquid  water  pos- 
sesses. A  quantity  of  heat  which  would  raise  a  pound  of 
ice  ten  degrees  in  temperature,  would  raise  a  pound  of  wa- 
ter only  five  degrees.  Further,  to  simply  liquefy  a  mass 
of  ice,  an  enormous  amount  of  heat  is  necessary,  this  heat 
being  so  utterly  absorbed  or  rendered  '  latent '  as  to  make 
no  impression  upon  the  thermometer.  The  question  of 
4  latent  heat '  shall  be  fully  discussed  in  a  future  lecture ; 
what  I  am  desirous  of  impressing  on  you  at  present  is,  that 
liquid  water,  at  its  freezing  temperature,  possesses  a  vastly 
greater  amount  of  heat  than  ice  at  the  same  temperature. 

Davy  reasoned  thus :  c  If  I,  by  friction,  liquefy  ice,  I 
produce  a  substance  which  contains  a  far  greater  absolute 
amount  of  heat  than  the  ice ;  and,  hi  this  case,  it  cannot, 
with  any  show  of  reason,  be  afiirmed  that  I  merely  render 
sensible  the  heat  hidden  in  the  ice,  for  that  quantity  is  only 
a  small  fraction  of  the  heat  contained  in  the  water.'  He 

*  Works  of  Sir  H.  Davy,  vol.  ii,,  p.  11. 


FUSION  OF  ICE  BY  FRICTION.  41 

made  the  experiment,  and  liquefied  the  ice  by  pure  friction ; 
and  the  result  has  been  regarded  as  the  first  which  proved 
the  immateriality  of  heat. 

When  a  hammer  strikes  a  bell,  the  motion  of  the  ham- 
mer is  arrested,  but  its  force  is  not  destroyed;  it  has 
thrown  the  bell  into  vibrations,  which  affect  the  auditory 
nerves  as  sound.  So,  also,  when  our  sledge  hammer  de- 
scended upon  our  lead  bullet,  the  descending  motion  of  the 
sledge  was  arrested :  but  it  was  not  destroyed.  Its  motion 
was  transferred  to  the  atoms  of  the  lead,  and  announced  it- 
self'to  the  proper  nerves  as  heat.  The  theory,  then,  which 
Rumford  so  powerfully  advocated,  and  Davy  so  ably  sup- 
ported,* was,  that  heat  is  a  kind  of  molecular  motion  ;  and 
that,  by  friction,  percussion,  or  compression,  this  motion 
may  be  generated,  as  well  as  by  combustion.  This  is  the 
theory  which  must  gradually  develope  itself  during  these 
lectures,  until  your  minds  attain  to  perfect  clearness  re- 
garding it.  And,  remember,  we  are  entering  a  jungle,  and 
must  not  expect  to  find  our  way  clear.*  We  are  striking 
into  the  brambles  in  a  random  fashion  at  first ;  but  we  shall 
thus  become  acquainted  with  the  general  character  of  our 
work,  and,  with  due  persistence,  shall,  I  trust,  cut  through 
all  entanglement  at  last. 

In  our  first  lecture  I  showed  you  the  effect  of  projecting 
a  current  of  compressed  air  against  the  face  of  the  thermo- 
electric pile.  You  saw  that  the  instrument  was  chilled  by 
the  current  of  air.  £Tow,  heat  is  known  to  be  developed 
when  air  is  compressed ;  and,  since  last  Thursday,  I  have 

*  In  Davy's  first  scientific  memoir,  he  calls  heat  a  repulsive  motion, 
which  he  says  may  be  augmented  in  various  ways.  *  First,  by  the  trans- 
mutation of  mechanical  into  repulsive  motion ;  that  is,  by  friction  or  per- 
cussion. In  this  case  the  mechanical  motion  lost  by  the  masses  of  mat- 
ter in  friction  is  the  repulsive  motion  gained  by  their  corpuscles : '  an 
extremely  remarkable  passage.  I  have  given  further  extracts  from  this 
paper  in  the  Appendix  to  Lecture  III. 


42  LECTURE   H. 

been  asked  how  this  heat  was  disposed  of  in  the  case  of  the 
condensed  air.  Pray  listen  to  my  reply.  Supposing  the 
vessel  which  contained  the  compressed  air  to  be  formed  of 
a  substance  perfectly  impervious  to  heat,  and  supposing  all 
the  heat  developed  by  my  arm,  in  compressing  the  air,  to  be 
retained  within  the  vessel,  that  quantity  of  heat  would  be 
exactly  competent  to  undo  what  I  had  done  and  to  restore 
the  compressed  air  to  its  original  volume  and  temperature. 
But  this  vessel  v  (fig.  12),  is  not  impervious  to  heat,  and  it 
was  not  my  object  to  draw  upon  the  heat  developed  by  my 

Fig.  12. 


arm;  I  therefore,  after  condensing  the  air,  allowed  the 
vessel  to  rest,  till  all  the  heat  generated  by  the  condensa- 
tion had  been  dissipated,  and  the  temperature  of  the  air 
within  and  without  the  vessel  was  the  same.  When,  there- 
fore, the  air  rushed  out,  it  had  not  the  heat  to  draw  upon, 
which  had  been  developed  during  compression.  The  heat 
from  which  it  derived  its  elastic  force  was  only  sufficient 
to  keep  it  at  the  temperature  of  the  surrounding  air.  In 
doing  its  work  a  portion  of  this  heat,  equivalent  to  the 


FIRE   SYRINGE.  43 

work  done,  was  consumed,  and  the  issuing  air  was  conse- 
quently chilled.  Do  not  be  disheartened  if  this  reasoning 
should  not  appear  quite  clear  to  you.  We  are  now  in  com- 
parative darkness,  but  as  we  proceed  light  will  gradually 
appear,  and  irradiate  retrospectively  our  present  gloom. 

I  wish  now  to  make  evident  to  you  that  heat  is  devel- 
oped by  the  compression  of  air.    Here  is  a  strong  cylinder 
of  glass  T  u  (fig.  13),  accurately  bored,  and  quite  smooth 
within.    Into  it  this  piston  fits  air-tight,  so  that,  by  driving 
the  piston  down,  I  can  forcibly  compress  the  air 
underneath  it;    and  when  the  air  is  thus  com-    Fi°-13- 
pressed,   heat    is    suddenly  generated.     Let    me 
prove  this.    I  take  a  morsel  of  cotton  wool,  and 
wet  it  with  this  volatile  liquid,  the  bisulphide  of 
carbon.    I  throw  this  bit  of  wetted  cotton  into 
the  glass  syringe,  and  instantly  eject  it.     It  has 
left  behind  it  a  small  residue  of  vapour.     I  com- 
press the  air  suddenly,  and  you  see  a  flash  of  light 
within  the  syringe.    The  heat  developed  by  the 
compression  has    been  sufficient    to    ignite    the 
vapour.    It  is  not  necessary  to  eject  the  wetted 
cotton ;  I  replace  it  in  the  tube,  and  urge  the  pis- 
ton downwards  ;  you  see  the  flash  as  before.    If, 
with  this  narrow  glass  tube,  I  blow  out  the  fumes 
generated  by  the  combustion  of  the  vapour,  I  can, 
without  once  removing  the  cotton  from  the  syr- 
inge, repeat  the  experiment  twenty  times.* 

I  have  here  arranged  an  experiment  intended 
to  give  you  another  illustration  of  the  thermal 
effect  produced  in  air  by  its  own  mechanical  ac- 
tion.   Here  is  a  tin  tube,  stopped  at  both  ends,  and  con- 
nected with  this  air-pump.     The  tin  tube  is  at  present  full 
of  air,  and  I  bring  the  face  of  my  pile  up  against  the 

*  The  accident  which  led  to  this  form  of  the  experiment  is  referred  to 
In  the  Appendix  to  this  Lecture. 


4r  LECTUKE  II. 

curved  surface  of  the  tube.  The  instrument  declares  that 
the  face  of  the  pile  in  contact  with  the  tin  tube  has  been 
warmed  by  the  latter.  I  was  quite  prepared  for  this  result, 
having  reason  to  know  that  the  air  within  the  tube  is 
slightly  warmer  than  that  without.  Now,  what  you  are  to 
observe  is  this : — My  assistant  shall  work  the  pump ;  the 
cylinders  of  the  machine  will  be  emptied  of  air,  and  the  air 
within  .this  tin  tube  will  be  driven  into  the  exhausted  cyl- 
inders by  its  own  elastic  force.  I  have  already  demon- 
strated the  chilling  effect  of  a  current  of  compressed  air  on 
the  thermo-electric  pile.  In  the  present  experiment  I  will 
not  examine  the  thermal  condition  of  the  current  at  all,  but 
of  the  vessel  in  which  the  work  has  been  performed.  As 
this  tube  is  exhausted  I  expect  to  see  the  needle,  which  is 
now  deflected  so  considerably  in  the  direction  of  heat, 
descend  to  zero,  and  pass  quite  up  to  90°  in  the  direction 
of  cold.  The  pump  is  now  in  action,  and  observe  the  re- 
sult. The  needle  falls  as  predicted,  and  its  advance  in  the 
direction  of  cold  is  only  arrested  by  its  concussion  against 
the  stops. 

Three  strokes  of  the  pump  suffice  to  chill  the  tube  so 
as  to  send  the  needle  up  to  90°  ;  *  let  it  now  come  to  rest. 
It  would  require  more  time  than  we  can  afford  to  allow  the 
tube  to  assume  the  temperature  of  the  air  around  it ;  but 
the  needle  is  now  sensibly  at  rest  at  a  good  distance  on 
the  cold  side  of  zero.  I  will  now  allow  a  quantity  of  air 
to  enter  the  tube,  equal  to  that  which  was  removed  from 
it  a  moment  ago  by  the  air-pump.  I  can  turn  on  this  cock, 
the  air  will  enter,  and  each  of  its  atoms  will  hit  the  inner 
surface  of  the  tube  like  a  projectile.  The  mechanical  mo- 
tion of  the  atom  will  be  thereby  annihilated,  but  an  amount 

*  The  galvanometer  used  in  this  experiment  was  that  which  I  employ 
in  my  original  researches :  it  is  an  exceedingly  delicate  one.  When  intro- 
duced in  the  lectures  its  dial  was  illuminated  by  the  electric  light ;  and  an 
image  of  it,  two  feet  in  diameter,  was  projected  on  the  screen. 


CONDENSATION  OF  AQUEOUS  VAPOUR.        45 

of  heat  equivalent  to  this  motion  will  be  generated.  Thus 
as  the  air  enters  it  will  develope  an  amount  of  heat  suffi- 
cient to  re-warm  the  tube,  to  undo  the  present  deflection, 
and  to  send  the  needle  up  on  the  heat  side  of  zero.  The 
air  is  now  entering,  and  you  see  the  effect :  the  needle 
moves,  and  goes  quite  up  to  90°  on  that  side  which  indi- 
cates the  heating  the  pile.* 

I  have  now  to  direct  your  attention  to  an  interesting 
effect  connected  with  this  chilling  of  the  air  by  rarefaction. 
I  place  over  the  plate  of  the  air  pump  a  large  glass  receiver, 
which  is  now  filled  with  the  air  of  this  room.  This  air, 
and,  indeed,  all  air,  unless  it  be  dried  artificially,  contains 
a  quantity  of  aqueous  vapour  which,  as  vapour,  is  perfectly 
invisible.  A  certain  temperature  is  requisite  to  maintain 
the  vapour  in  this  invisible  state,  and  if  the  air  be  chilled 
so  as  to  bring  it  below  this  temperature,  the  vapour  will 
instantly  condense,  and  form  a  visible  cloud.  Such  a  cloud, 
which  you  will  remember  is  not  vapour,  but  liquid  water 
in  a  state  of  fine  division,  will  form  within  this  glass  vessel 
R  (fig.  14),  when  the  air  is  pumped  out  of  it;  and  to  make 
this  effect  visible  to  everybody  present,  to  those  right  and 
left  of  me,  as  well  as  to  those  in  front,  these  six  little  gas 
jets  are  arranged  in  a  semicircle,  which  half  surrounds  the 
receiver.  Each  person  present  sees  one  or  more  of  these 

*  In  this  experiment  a  mere  line  along  the  surface  of  the  tube  was  in 
contact  with  the  face  of  the  pile,  and  the  heat  had  to  propagate  itself 
through  the  tin  envelope  to  reach  the  instrument.  Previous  to  adopting 
this  arrangement  I  had  the  tube  pierced,  and  a  separate  pile,  with  its  naked 
face  turned  inwards,  cemented  air-tight  into  the  orifice.  The  pile  came 
thus  into  direct  contact  with  the  air,  and  its  entire  face  was  exposed  to  the 
action.  The  effects  thus  obtained  were  very  large ;  sufficient,  indeed,  to 
swing  the  needle  quite  round.  My  desire  to  complicate  the  subject  as  little 
as  possible  induced  me  to  abandon  the  cemented  pile,  and  to  make  use 
of  the  instrument  with  which  my  audience  had  already  become  familiar. 
With  the  arrangement  actually  adopted  the  effects  were,  moreover,  so 
large,  that  I  drew  only  on  a  portion  of  my  power  to  produce  them. 


46  LECTURE  n. 

jets  on  looking  through  the  receiver,  and  when  the  cloud 
forms,  the  dimness  which  it  produces  will  at  once  declare 
its  presence.  The  pump  is  now  quickly  worked ;  a  very  few 
strokes  suffice  to  precipitate  the  vapour ;  there  it  spreads 
throughout  the  entire  receiver,  and  many  of  you  see  a  col- 
rig.  14. 


ouring  of  the  cloud,  as  the  light  shines  through  it,  similar 
to  that  observed  sometimes,  on  a  large  scale,  around  the 
moon.  When  I  allow  the  air  to  re-enter  the  vessel,  it  is 
heated,  exactly  as  in  the  experiment  with  our  tin  tube ; 
the  cloud  melts  away,  and  the  perfect  transparency  of  the 
air  within  the  receiver  is  restored.  Again  I  exhaust  and 
again  the  cloud  forms ;  once  more  the  air  enters  and  the 
cloud  disappears  ;  the  heat  developed  being  more  than  suffi- 
cient to  preserve  it  in  the  state  of  pure  vapour.* 

Sir  Humphry  Davy  refers,  in  his  '  Chemical   Philos- 
ophy,' to  a  machine  at  Schemnitz,  in  Hungary,  in  which 
air  was  compressed  by  a  column  of  water  260  feet  in 
height.    "When  a  stopcock  was  opened,  so  as  to  allow  the 
*  See  Note  (1)  at  the  end  of  this  Lecture. 


FRICTION  AGAINST  SPACE.  47 

air  to  escape,  a  degree  of  cold  was  produced  which  not 
only  precipitated  the  aqueous  vapour  diffused  in  the  air, 
but  caused  it  to  congeal  in  a  shower  of  snow,  while  the 
pipe  from  which  the  air  issued  became  bearded  with  icicles. 
'  Dr.  Darwin,'  writes  Davy,  '  has  ingeniously  explained  the 
production  of  snow  on  the  tops  of  the  highest  mountains, 
by  the  precipitation  of  vapour  from  the  rarefied  air  which 
ascends  from  plains  and  valleys.  The  Andes,  placed  almost 
under  the  line,  rise  in  the  midst  of  burning  sands ;  about 
the  middle  height  is  a  pleasant  and  mild  climate ;  the  sum- 
mits are  covered  with  unchanging  snows.' 

I  would  now  request  your  attention  to  another  experi- 
ment, in  which  heat  will  be  developed  by  what  must  ap- 
pear to  many  of  you  a  very  mysterious  agency,  and,  indeed, 
the  most  instructed  amongst  us  know,  in  reality,  very  little 
about  the  subject.  I  wish  to  develope  heat  by  what  might 
be  regarded  as  friction  against  pure  space.  And  indeed  it 
may  be,  and  probably  is,  due  to  a  kind  of  friction  against 
that  inter-stellar  medium,  to  which  we  shall  have  occasion 
to  refer  more  fully  by  and  by. 

I  have  here  a  mass  of  iron — part  of  a  link  of  a  huge 
chain  cable — which  is  surrounded  by  these  multiple  coils 
of  copper  wire  c  c  (fig.  15),  and  which  I  can  instantly  con- 
vert into  a  powerful  magnet  by  sending  an  electric  current 
through  the  wire.  You  see,  when  thus  excited,  how  pow- 
erful it  is.  This  poker  clings  to  it,  and  these  chisels, 
screws,  and  nails  cling  to  the  poker.  Turned  upside  down, 
this  magnet  will  hold  a  half  hundred  weight  attached  to 
eacn  of  its  poles,  and  probably  a  score  of  the  heaviest  peo- 
ple in  this  room,  if  suspended  from  the  weights.  At  the 
proper  signal  my  assistant  will  interrupt  the  electric  cur- 
rent : — '  Break ! '  The  iron  falls,  and  all  the  magic  disap- 
pears :  the  magnet  now  is  mere  common  iron.  At  the 
ends  of  the  magnet  I  place  two  pieces  of  iron  p  P — mov- 
able poles,  as  they  are  called — which,  when  the  magnet  is 


APPARENT   VISCOSITY   OF   MAGNETIC   FIELD.  49 

unexcited,  I  can  bring  within  any  required  distance  of  each 
other.  When  the  current  passes,  these  pieces  of  iron  vir- 
tually form  parts  of  the  magnet.  Between  them  I  will 
place  a  substance  which  the  magnet,  even  when  exerting 
its  utmost  power,  is  incompetent  to  attract.  Tliis  substance 
is  simply  a  piece  of  silver — in  fact,  a  silver  medal.  I  bring 
it  close  to  the  excited  magnet ;  no  attraction  ensues.  In- 
deed, what  little  force — and  it  is  so  little  as  to  be  utterly 
insensible  in  these  experiments — the  magnet  really  exerts 
upon  the  silver,  is  repulsive  instead  of  attractive. 

Well,  I  suspend  this  medal  between  the  poles  P  p  of 
the  magnet,  and  excite  the  latter.  The  medal  hangs  there ; 
it  is  neither  attracted  nor  repelled,  but  if  I  seek  to  move  it 
I  encounter  resistance.  To  turn  the  medal  round  I  must 
overcome  this  resistance ;  the  silver  moves  as  if  it  were 
surrounded  by  a  viscous  fluid.  This  curious  effect  may  also 
be  rendered  manifest,  thus :  I  have  here  a  rectangular  plate 
of  copper,  and  if  I  cause  it  to  pass  quickly  to  and  fro  like 
a  saw  between  the  poles,  when  their  points  are  turned 
towards  it,  I  seem,  though  I  can  see  nothing,  to  be  sawing 
through  a  mass  of  cheese  or  butter.*  Nothing  of  this  kind 
is  noticed  when  the  magnet  is  not  active  :  the  copper  saw 
then  encounters  nothing  but  the  infinitesimal  resistance  of 
the  air.  Thus  far  you  have  been  compelled  to  take  my 
statements  for  granted,  but  I  have  arranged  an  experiment 
which  will  make  this  strange  action  of  the  magnet  on  the 
silver  medal,  strikingly  manifest  to  everybody  present. 

Above  the  suspended  medal,  and  attached  to  it  by  a  bit 
of  wire,  I  have  a  little  reflecting  pyramid  M,  formed  of  four 
triangular  pieces  of  looking-glass  ;  both  the  medal  and  the 
reflector  are  suspended  by  a  thread  which  was  twisted  in 
its  manufacture,  and  which  will  untwist  itself  when  the 
weight  it  sustains  is  set  free.  I  place  our  electric  lamp  so 

*  An  experiment  of  Faraday's. 


50  LECTUKE   II. 

as  to  cast  a  strong  beam  of  light  on  this  little  pyramid : 
you  see  these  long  spokes  of  light  passing  through  the 
dusty  air  of  the  room  as  the  mirror  turns. 

Let  us  start  it  from  a  state  of  rest.  You  now  see  the 
beam  passing  through  the  room  and  striking  against  the 
white  wall.  As  the  mirror  commences  to  rotate,  the  patch 
of  light  moves,  at  first  slowly,  over  the  wall  and  ceiling. 
But  the  motion  quickens,  and  now  you  can  no  longer  see 
the  distinct  patches  of  light,  but  instead  of  them  you  have 
this  splendid  luminous  band  fully  twenty  feet  in  diameter 
drawn  upon  the  wall  by  the  quick  rotation  of  the  reflected 
beams.  At  the- word  of  command  the  magnet  will  be  ex- 
cited, and  the  motion  of  the  medal  will  be  instantly 
stopped.  4  Make  ! '  See  the  effect :  the  medal  seems 
struck  dead  by  the  excitement  of  the  magnet,  the  band 
suddenly  disappears,  and  there  you  have  the  single  patch 
of  light  upon  the  wall.  This  strange  mechanical  effect  is 
produced  without  any  visible  change  in  the  space  between 
the  two  poles.  Observe  the  slight  motion  of  the  image  on 
the  wall :  the  tension  of  the  string  is  struggling  with  an 
unseen  antagonist  and  producing  that  slight  motion.  It  is 
such  as  would  be  produced  if  the  medal,  instead  of  being 
surrounded  by  air,  were  immersed  in  a  pot  of  thick  trea- 
cle. I  destroy  the  magnetic  power,  and  the  viscous  charac- 
ter of  the  space  between  the  poles  instantly  disappears ;  the 
medal  begins  to  twirl  as  before ;  there  are  the  revolving 
beams,  and  there  is  now  the  luminous  band.  I  again  ex- 
cite the  magnet :  the  beams  are  struck  motionless,  and  >the 
band  disappears. 

By  the  force  of  my  hand  I  can  overcome  this  resistance 
and  turn  the  medal  round ;  but  to  turn  it  I  must  expend 
force.  Where  does  that  force  go  ?  It  is  converted  into 
heat.  The  medal,  if  forcibly  compelled  to  turn,  will  be- 
come heated.  Many  of  you  are  acquainted  with  the  grand 
discovery  of  Faraday,  that  electric  currents  are  developed 


HEAT   GENEKATED   IN  MAGNETIC   IpSJX  51 

where  a  conductor  of  electricity  is  set  in  motion  between 
the  poles  of  a  magnet.  We  have  these  currents  doubtless 
here,  and  they  are  competent  to  heat  the  medal.  But  what 
are  these  currents  ?  how  are  they  related  to  the  space  be- 
tween the  magnetic  poles — how  to  the  force  of  my  arm 
which  is  expended  in  their  generation  ?  We  do  not  yet 
know,  but  we  shall  know  by  and  by.  It  does  not  in  the 
least  lessen  the  interest  of  the  experiment  if  the  force  of 
my  arm,  previous  to  appearing  as  heat,  appears  in  another 
form — in  the  form  of  electricity.  The  ultimate  result  is 
the  same  :  the  heat  developed  ultimately  is  the  exact  equiv- 
alent of  the  quantity  of  strength  required  to  move  the 
medal  in  the  excited  magnetic  field. 

I  wish  now  to  show  you  the  developement  of  heat  by 
this  action.  I  have  here  a  solid  metal  cylinder,  the  core 
of  which  is,  however,  composed  of  a  metal  more  easily 
melted  than  its  outer  case.  The  outer  case  is  copper,  and 
this  is  filled  by  a  hard  but  fusible  alloy.  I  set  this  cylinder 
upright  between  the  conical  poles  p  P  (fig.  16)  of  the  mag- 
rig.  16. 


net.  A  string  s  s  passes  from  the  cylinder  to  a  whirling 
table,  and  by  turning  the  latter  the  cylinder  is  caused  to 
spin  round.  It  might  turn  till  doomsday,  as  long  as  the 
magnet  remains  unexcited,  without  producing  the  effect 
sought ;  but  when  the  magnet  is  in  action,  I  hope  to  be 
able  to  develope  an  amount  of  heat  sufficient  to  melt  the 
core  of  that  cylinder,  and,  if  successful,  I  will  pour  the 
liquid  metal  out  before  you.  Two  minutes  will  suffice  for 
this  experiment.  The  cylinder  is  now  rotating,  and  its 


52  LECTURE   II. 

upper  end  is  open.  I  shall  leave  it  thus  open  until  the 
liquid  inetal  is  seen  spattering  over  the  poles  of  the  mag- 
net. I  already  see  the  metallic  spray,  though  a  minute  has 
scarcely  elapsed  since  the  commencement  of  the  experi- 
ment. I  now  stop  the  motion  for  a  moment,  and  cork  up 
the  end  of  the  cylinder,  so  as  to  prevent  the  scattering 
about  of  the  metal.  Let  the  action  continue  for  half  a  min- 
ute lon'ger ;  the  entire  mass  of  the  core  is,  I  am  persuaded, 
now  melted.  I  withdraw  the  cylinder,  remove  the  cork, 
and  here  is  the  liquefied  mass,  which  I  thus  pour  out  before 
you.* 

It  is  now  time  to  consider  more  closely  than  we  have 
hitherto  done,  the  relation  of  the  heat  developed  by  me- 
chanical action  to  the  force  which  produces  it.  Doubtless 
this  relation  floated  in  many  minds  before  it  received  either 
distinct  enunciation  or  experimental  proof.  Those  who 
reflect  on  vital  processes — oh  the  changes  which  occur  in 
the  animal  body — and  the  relation  of  the  forces  involved  in 
food,  to  muscular  force,  are  led  naturally  to  entertain  the 
idea  of  interdependence  between  these  forces.  It  is,  there- 
fore, not  a  matter  of  surprise  that  the  man  who  first  raised 
the  idea. of  the  equivalence  between  heat  and  mechanical 
energy  to  philosophic  clearness  in  his  own  mind,  was  a 
physician.  Dr.  Mayer,  of  Heilbronn,  in  Germany,  enunci- 
ated f  the  exact  relation  which  subsists  between  heat  and 
work,  giving  the  number  which  is  now  known  as  the  '  me- 
chanical equivalent  of  heat,'  and  following  up  the  state- 
ment of  the  principle  by  its  fearless  application. J  It  is, 

*  The  dovelopement  of  heat  by  causing  a  conductor  to  revolve  between 
the  poles  of  a  magnet  was  first  effected  by  Mr.  Joule  (Phil.  Mag.  vol.  xxiii. 
3rd  Series,  year  1843,  pp.  355  and  439),  and  his  experiment  was  after- 
wards revived  in  a  striking  form  by  M.  Foucault.  The  artifice  above 
described,  of  fusing  the  core  out  of  the  cylinder,  renders  the  experiment 
very  effective  in  the  lecture-room. 

f  In  1842.     Sec  Note  (2)  at  the  end  of  this  Lecture. 
See  Lectures  III.  and  XIII. 


MAYER  AND  JOULE.  53 

however,  to  Mr.  Joule,  of  Manchester,  that  we  are  almost 
wholly  indebted  for  the  experimental  treatment  of  this  im- 
portant subject.  Entirely  independent  of  Mayer,  with  his 
mind  firmly  fixed  upon  a  principle,  and  undismayed  by  the 
coolness  with  which  his  first  labours  appear  to  have  been 
received,  he  persisted  for  years  in  his  attempts  to  prove 
the  invariability  of  the  relation  which  subsists  between 
heat  and  ordinary  mechanical  force.  He  placed  water  in  a 
suitable  vessel,  and  agitated  that  water  by  paddles,  driven 
by  measurable  forces,  and  determined  both  the  amount  of 
heat,  developed  by  the  stirring  of  the  liquid,  and  the 
amount  of  labour  expended  in  the  process.  He  did  the 
same  with  mercury  and  with  sperm  oil.  He  also  caused 
disks  of  cast  iron  to  rub  against  each  other,  and  measured 
the  heat  produced  by  their  friction,  and  the  force  expended 
in  overcoming  it.  He  also  urged  water  through  capillary 
tubes,  and  determined  the  amount  of  heat  generated  by  the 
friction  of  the  liquid  against  the  sides  of  the  tubes.  And 
the  results  of  his  experiments  leave  no  shadow  of  doubt 
upon  the  mind  that,  under  all  circumstances,  the  quantitjr 
of  heat  generated  by  the  same  amount  of  force  is  fixed  and 
invariable.  A  given  amount  of  force,  in  causing  the  iron 
disks  to  rotate  against  each  other,  produced  precisely  the 
same  amount  of  heat,  as  when  it  was  applied  to  agitate 
water,  mercury,  or  sperm  oil.  Of  course,  at  the  end  of  an 
experiment,  the  temperatures  in  the  respective  cases  would 
be  very  different ;  that  of  the  water,  for  example,  would 
be  ^oth  of  the  temperature  of  the  mercury,  because,  as  we 
already  know,  the  capacity  of  water  for  heat  is  30  times 
that  of  mercury.  Mr.  Joule  took  this  into  account  in  re- 
ducing his  experiments,  and  found,  as  I  have  stated,  that, 
however  the  temperatures  might  differ,  in  consequence  of 
the  different  capacity  of  heat  for  the  substances  employed, 
the  absolute  amount  of  heat  generated  by  the  same  expend- 
iture of  power,  was  in  all  cases  the  same. 


54  LECTURE   II. 

In  this  way  it  was  found  that  the  quantity  of  heat 
which  would  raise  one  pound  of  water  one  degree  Fahr.  in 
temperature,  is  exactly  equal  to  what  would  be  generated 
if  a  pound  weight,  after  having  fallen  through  a  height  of 
772  feet,  has  its  moving  force  destroyed  by  collision  with 
the  earth.  Conversely,  the  amount  of  heat  necessary  to 
raise  a  pound  of  water  one  degree  in  temperature,  would^ 
if  all  applied  mechanically,  be  competent  to  raise  a  pound 
weight  772  feet  high,  or  it  would  raise  772  Ibs.  one  foot 
high.  The  term  '  foot-pound '  has  been  introduced  to  ex- 
press, in  a  convenient  way,  the  lifting  of  one  pound  to  the 
height  of  a  foot.  Thus  the  quantity  of  heat  necessary  to 
raise  the  temperature  of  a  pound  of  water  one  degree  being 
taken  as  a  standard,  772  foot-pounds  constitute  what  is 
called  the  mechanical  equivalent  of  heat.* 

In  order  to  imprint  upon  your  minds  the  thermal  effect 
produced  by  a  body  falling  from  a  height,  I  will  go  through 
the  experiment  of  allowing  a  lead  ball  to  fall  from  our 
ceiling  upon  this  floor.  The  lead  ball  is  at  the  present  mo- 
ment slightly  colder  than  the  air  of  this  room.  I  prove 
this  by  bringing  it  in  contact  with  the  thermo-electric  pile, 
and  showing  you  that  the  deflection  of  the  needle  indicates 
cold.  Here  on  the  floor  I  have  placed  a  slab  of  iron,  on 
which  I  intend  the  lead  to  fall,  and  which,  you  observe,  is 
also  cooler  than  the  air  of  the  room.  At  the  top  of  the 
house  I  have  an  assistant,  who  wilj.  heave  up  the  ball  after 
I  have  attached  it  to  this  string.  He  will  not  touch  the 
ball,  nor  will  he  allow  it  to  touch  anything  else.  He  will 
now  let  it  go ;  it  falls,  and  is  received  upon  the  plate  of 
iron.  The  height  is  too  small  to  get  much  heat  by  a  single 
fall ;  I  will  therefore  have  the  ball  drawn  up  and  dropped 
three  or  four  times  in  succession.  Observe,  there  is  a 
length  of  covered  wire  attached  to  the  ball,  by  which  I  lift 
it,  so  that  my  hand  never  comes  near  the  ball.  There  is 
the  fourth  collision,  and  I  think  I  may  now  examine  the 
*  See  Note  (3)  at  the  end  of  this  Lecture. 


'MECHANICAL  EQUIVALENT'  OF  HEAT.  55 

temperature  of  the  lead.  I  place  the  ball,  which  at  the 
commencement  was  cold,  again  upon  the  pile,  and  the  im- 
mediate deflection  of  the  needle  in  the  opposite  direction, 
declares  that  now  the  ball  is  heated ;  this  heat  is  due  en- 
tirely to  the  destruction  of  the  moving  energy  which  the 
ball  possessed  when  it  struck  the  plate  of  iron.  According 
to  our  theory,  the  common  mechanical  motion  of  the  ball 
as  a  mass,  has  been  transferred  to  the  atoms  of  the  mass, 
producing  among  them  the  agitation  which  we  call  heat. 

What  was  the  total  amount  of  heat  thus  generated  ? 
The  space  fallen  through  by  the  ball  in  each  experiment  is 
twenty-six  feet.  The  heat  generated  is  proportional  to  the 
height  through  which  the  body  falls.  Now  a  ball  of  lead, 
in  falling  through  772  feet,  would  generate  heat  sufficient 
to  raise  its  own  temperature  30°,  its  '  capacity '  being  ^Vth 
of  that  of  water :  hence,  in  falling  through  26  feet,  which  is 
in  round  numbers  ^V  of  772,  the  heat  generated  would,  if 
all  concentrated  in  the  lead,  raise  its  temperature  one  de- 
gree. This  is  the  amount  of  heat  generated  by  a  single 
descent  of  the  ball,  and  four  times  this  amount  would,  of 
course,  be  generated  by  four  descents.  The  heat  generated 
is  not,  however,  all  concentrated  in  the  ball ;  it  is  divided 
between  the  ball  and  the  iron  on  which  it  falls. 

It  is  needless  to  say,  that  if  motion  be  imparted  to  a 
body  by  other  means  than  gravity,  the  destruction  of  this 
motion  also  produces  heat.  A  rifle  bullet,  when  it  strikes 
a  target,  is  intensely  heated.  The  mechanical  equivalent 
of  heat  enables  us  to  calculate  with  the  utmost  accuracy 
the  amount  of  heat  generated  by  the  bullet,  when  its  ve- 
locity is  known.  This  is  a  point  worthy  of  our  attention, 
and  in  dealing  with  it  I  will  address  myself  to  those  of  my 
audience  who  are  unacquainted  even  with  the  elements  of 
mechanics.  Everybody  knows  that  the  greater  the  height 
is  from  which  a  body  falls,  the  greater  is  the  force  with 
which  it  strikes  the  earth,  and  that  this  is  entirely  due  to 


56  LECTUBE   H. 

the  greater  velocity  imparted  to  the  ball,  in  falling  from  the 
greater  height.  The  velocity  imparted  to  the  body  is  not, 
however,  proportional  to  the  height  from  which  it  falls. 
If  the  height  be  augmented  four-fold,  the  velocity  is  aug- 
mented only  two-fold ;  if  the  height  be  augmented  nine- 
fold, the  velocity  is  augmented  only  three-fold;  if  the 
height  be  augmented  sixteen-fold,  the  velocity  is  augmented 
only  four-fold ;  or,  expressed  generally,  the  height  aug- 
ments in  the  same  proportion  as  the  square  of  the  velocity. 

But  the  heat  generated  by  the  collision  of  the  falling 
body  increases  simply  as  the  height;  consequently,  the 
heat  generated  increases  as  the  square  of  the  velocity. 

If,  therefore,  we  double  the  velocity  of  a  projectile,  we 
augment  the  heat  generated,  when  its  moving  force  is  de- 
stroyed, four-fold ;  if  we  treble  its  velocity,  we  augment  the 
heat  nine-fold  ;  if  we  quadruple  the  velocity,  we  augment 
the  heat  sixteen-fold  ;  and  so  on. 

The  velocity  imparted  to  a  body  by  gravity  in  falling 
through  772  feet  is,  in  round  numbers,  223  feet  a  second, 
that  is  to  say,  immediately  before  the  Ibody  strikes  the 
earth,  this  is  its  velocity.  Six  times  this  quantity  or  1,338 
feet  a  second,  would  not  be  an  inordinate  velocity  for  a 
rifle  bullet. 

But  a  rifle  bullet,  if  formed  of  lead,  moving  at  a  ve- 
locity of  223  feet  a  second,  would  generate,  on  striking  a 
target  an  amount  of  heat  which,  if  concentrated  in  the  bul- 
let, would  raise  its  temperature  30°  ;  with  6  times  this  ve- 
locity it  will  generate  36  times  this  amount  of  heat ;  hence 
36  times  30,  or  1,080°,  would  represent  the  augmentation 
of  temperature  of  a  rifle  ball  on  striking  a  target  with  a 
velocity  of  1,338  feet  a  second,  if  all  the  heat  generated 
were  confined  to  the  bullet  itself.  This  amount  of  heat 
would  be  far  more  than  sufficient  to  fuse  the  lead ;  but  in 
reality  a  portion  only  of  the  heat  generated  is  lodged  in  the 
ball,  the  total  amount  being  divided  between  it  and  the 


RELATION  OF  HEAT  TO  VELOCITY.          57 

target.  Were  the  ball  iron  instead  of  lead,  the  heat  gen- 
erated, under  the  conditions  supposed,  would  be  competent 
to  raise  the  temperature  of  the  ball  only  by  about  £rd  of 
1,080°,  because  the  capacity  of  iron  for  heat  is  about  three 
times  that  of  lead. 

From  these  considerations  I  think  it  is  manifest  that  if 
we  know  the  velocity  and  Aveight  of  any  projectile,  we  can 
calculate,  with  ease,  the  amount  of  heat  developed  by  the 
destruction  of  its  moving  force.  For  example,  knowing, 
as  we  do,  the  weight  of  the  earth,  and  the  velocity  with 
which  it  moves  through  space,  a  simple  calculation  would 
enable  us  to  determine  the  exact  amount  of  heat  which 
would  be  developed,  supposing  the  earth  to  be  stopped  in 
her  orbit.  We  could  tell,  for  example,  the  number  of  de- 
grees which  this  amount  of  heat  would  impart  to  a  globe 
of  water  equal  to  the  earth  in  size.  Mayer  and  Helmholtz 
have  made  this  calculation,  and  found  that  the  quantity  of 
heat  generated  by  this  colossal  shock  would  be  quite  suffi- 
cient, not  only  to  fuse  the  entire  earth,  but  to  reduce  it,  in 
great  part,  to  vapour.  Thus,  by  the  simple  stoppage  of 
the  earth  in  its  orbit  '  the  elements '  might  be  caused  '  to 
melt  with  fervent  heat.'  The  amount  of  heat  thus  devel- 
oped would  be  equal  to  that  derived  from  the  combustion 
of  fourteen  globes  of  coal,  each  equal  to  the  earth  in  mag- 
nitude. And  if,  after  the  stoppage  of  its  motion,  the  earth 
should  fall  into  the  sun,  as  it  assuredly  would,  the  amount 
of  heat  generated  by  the  blow  would  be  equal  to  that  de- 
veloped by  the  combustion  of  5,600  worlds  of  solid  carbon. 

Knowledge,  such  as  that  which  you  now  possess,  has 
caused  philosophers,  in  speculating  on  the  mode  in  which 
the  sun  is  nourished,  and  his  supply  of  light  and  heat  kept 
up,  to  suppose  the  heat  and  light  to  be  caused  by  the 
showering  down  of  meteoric  matter  irpon  the  sun's  sur- 
face.* Some  philosophers  suppose  the  Zodiacal  Light  to 

*  Mayer  propounded  this  hypothesis  in  184S,  and  worked  it  fully  out. 
3* 


58  LECTURE   II. 

be  a  cloud  of  meteorites,  and  from  it,  it>  is  imagined,  the 
showering  meteoric  matter  may  be  derived.  Now,  what- 
ever be  the  value  of  this  speculation,  it  is  to  be  borne  in 
mind  that  the  pouring  down  of  meteoric  matter,  in  the 
way  indicated,  would  be  competent  to  produce  the  light 
and  heat  of  the  sun.  With  regard  to  the  probable  truth  or 
fallacy  of  the  theory,  it  is  not  necessary  that  I  should  offer 
an  opinion  ;  I  would  only  say  that  the  theory  deals  with  a 
cause  which,  if  in  sufficient  operation,  would  be  competent 
to  produce  the  effects  ascribed  to  it. 

Let  me  now  pass  from  the  sun  to  something  less, — in 
fact,  to  the  opposite  pole  of  nature.  And  here  that  divine 
power  of  the  human  intellect  which  annihilates  mere  mag- 
nitude in  its  dealings  with  law,  comes  conspicuously  into 
play.  Our  reasoning  applies  not  only  to  suns  and  planets, 
but  equally  so  to  the  very  ultimate  atoms  of  which  matter 
is  composed.  Most  of  you  know  the  scientific  history  of 
the  diamond,  that  Newton,  antedating  intellectually  the 
discoveries  of  modern  chemistry,  pronounced  it  to  be  an 
unctuous  or  combustible  substance.  Everybody  now 
knows  that  this  brilliant  gem  is  composed  of  the  same 
substance  as  common  charcoal,  graphite,  or  plumbago.  A 
diamond  is  pure  carbon,  and  carbon  burns  in  oxygen.  I 
have  here  a  diamond,  held  fast  in  a  loop  of  platinum  wire ; 
I  will  heat  the  gem  to  redness  in  this  flame,  and  then 
plunge  it  into  this  jar,  which  contains  oxygen  gas.  See 
how  it  brightens  on  entering  the  jar  of  oxygen,  and  now  it 
glows,  like  a  little  terrestrial  star,  with  a  pure  white  light. 
How  are  we  to  figure  the  action  here  going  on  ?  Exactly 
as  you  would  present  to  your  minds  the  conception  of  me- 
teorites showering  down  upon  the  sun.  The  conceptions 


It  was  afterwards  enunciated  independently  by  Mr.  Waterston,  and  devel- 
oped by  Professor  William  Thomson  (Transactions  of  the  Royal  Soc.  of 
Edinb.,  1853).  See  Lecture  XII. 


f 


TIIEOEY   OF   COMBUSTION.  50 

are,  in  quality,  the  same,  and  to  the  intellect  the  one  is 
not  more  difficult  than  the  other.  You  are  to  figure  the 
atoms  of  oxygen  showering  against  this  diamond  on  all 
sides.  They  are  urged  towards  it  by  what  is  called  chemi- 
cal affinity,  but  this  force,  made  clear,  presents  itself  to  the 
mind  as  pure  attraction,  of  the  same  mechanical  quality,  if 
I  may  use  the  term,  as  gravity.  Every  oxygen  atom,  as  it 
strikes  the  surface,  and  has  its  motion  of  translation  de- 
stroyed by  its  collision  with  the  carbon,  assumes  the  motion 
which  we  call  heat :  and  this  heat  is  so  intense,  the  attrac- 
tions exerted  at  these  molecular  distances  are  so  mighty, 
that  the  crystal  is  kept  white-hot,  and  the  compound, 
formed  by  the  union  of  its  atoms  with  those  of  the  oxy- 
gen, flies  away  as  carbonic  acid  gas. 

Let  us  now  pass  on  from  the  diamond  to  ordinary 
flame.  I  have  here  a  burner  from  which  I  can  obtain  an 
ignited  jet  of  gas.  Here  is  the  flame :  what  is  its  constitu- 
tion ?  Within  the  flame  we  have  a  core  of  pure  unburnf 
gas,  and  outside  the  flame  we  have  the  oxygen  of  the  air. 
The  external  surface  of  the  core  of  gas  is  in  contact  with 
the  air,  and  here  it  is  that  the  atoms  clash  together  and 
produce  light  and  heat  by  their  collision.  But  the  exact 
constitution  of  the  flame  is  worthy  of  our  special  attention, 
and  for  our  knowledge  of  this  we  are  indebted  to  one  of 
Davy's  most  beautiful  investigations.  Coal-gas  is  what  we 
call  a  hydro-carbon  ;  it  consists  of  carbon  and  hydrogen  in 
a  state  of  chemical  union.  From  this  transparent  gas  es- 
cape the  soot  and  lampblack  which  we  notice  when  the 
combustion  of  the  gas  is  incomplete.  Soot  and  lampblack 
are  there  now,  but  they  are  compounded  with  other  sub- 
stances to  a  transparent  form.  Here,  then,  we  have  a  sur- 
face of  this  compound  gas,  in  presence  of  the  oxygen  of 
our  air ;  we  apply  heat,  and  the  attractions  are  instantly 
so  intensified  that  the  gas  bursts  into  flame.  The  oxygen 
has  a  choice  of  two  partners,  or,  if  you  like,  it  is  in  the 


60 


LECTUEE   II. 


Fig.  IT, 


presence  of  two  foes ;  it  closes  with  that  which  it  likes 
best,  or  hates  most  heartily,  as  the  case  may  be.  It  first 
closes  with  the  hydrogen,  and  sets  the  carbon  free.  Solid 
particles  of  carbon  thus  scattered  in  numbers  innumerable 
hi  the  midst  of  burning  matter,  are  raised  to  a  state  of  in- 
tense incandescence ;  they  become  white-hot,  and  mainly  to 
them  the  light  of  our  lamps  is  due.  The  carbon,  however, 
in  due  time,  closes  with  the  oxygen,  and  becomes,  or  ought 
to  become,  carbonic  acid ;  but  in  passing  from  the  hydro- 
gen with  which  it  was  first  combined,  to  the  oxygen,  with 

which  it  enters  into  final  union, 
it  exists,  for  a  time,  in  the  sin- 
gle state,  and,  as  a  bachelor,  it 
gives  us  all  the  splendour  of  its 
light. 

The  combustion  of  a  candle 
is  in  principle  the  same  as  that 
of  a  jet  of  gas.  Here  you  have 
a  rod  of  wax  or  tallow  (fig.  17), 
through  which  is  passed  the 
cotton  wick.  You  ignite  the 
wick ;  it  burns,  melts  the  tal- 
low at  its  base,  the  liquid  as- 
cends through  the  wick  by  cap- 
illary attraction,  it  is  converted  by  the  heat  into  vapour, 
and  this  vapour  is  a  hydro-carbon,  which  burns  exactly  like 
the  gas.  Here  also  you  have  unburnt  vapour  within,  com- 
mon air  without,  while  between  both  is  a  shell  which 
forms  the  battle-ground  of  the  clashing  atoms,  where  they 
develope  their  light  and  heat.  There  is  hardly  anything  in 
nature  more  beautiful  than  a  burning  candle  j  the  hollow 
basin  partially  filled  with  melted  matter  at  the  base  of  the 
wick,  the  creeping  up  of  the  liquid ;  its  vaporisation  ;  the 
structure  of  the  flame ;  its  shape,  tapering  to  a  point,  while 
converging  air-currents  rush  in  to  supply  its  needs.  Its 


STEUCTUKE   OF   FLAME.  61 

beauty,  its  brightness,  its  mobility,  have  made  it  a  favour- 
ite type  of  spiritual  essences,  and  its  dissection  by  Davy, 
far  from  diminishing  the  pleasure  with  which  we  look 
upon  a  flame,  has  rendered  it  more  than  ever  a  miracle  of 
beauty  to  the  enlightened  mind. 

You  ought  now  to  be  able  to  picture  clearly  before 
your  minds  the  structure  of  a  candle-flame.  You  ought  to 
see  the  unburnt  core  within  and  the  burning  shell  which 
envelopes  this  core.  From  the  core,  through  this  shell, 
the  constituents  of  the  candle  are  incessantly  passing  and 
escaping  to  the  surrounding  air.  In  the  case  of  a  candle 
you  have  a  hollow  cone  of  burning  matter.  Imagine  this 
cone  cut  across  horizontally;  you  would  then  expose  a 
burning  ring.  I  will  practically  cut  the  flame  of  a  candle 
thus  across.  I  have  here  a  piece  of  white  paper,  which  I 
will  bring  down  upon  the  candle ;  pressing  it  down  upon 
the  flame  until  it  almost  touches  the  wick.  Observe  the 
upper  surface  of  that  paper ;  it  becomes  charred,  but  how  ? 
Exactly  in  correspondence  with  the  burning  ring  of  the 
candle,  we  have  a  charred  ring  upon  the  paper  (fig.  18). 

Fig.  IS. 


I  might  operate  in  the  same  manner  with  a  jet  of  gas.  I 
will  do  so.  Here  is  the  ring  which  it  produces.  Within 
the  ring,  you  see,  there  is  no  charring  of  the  paper,  for  at 
this  place  the  unburnt  vapour  of  the  candle,  or  the  unburnt 
gas  of  the  jet,  impinges  against  the  surface,  and  no  charring 
can  be  produced. 

To  the  existence,  then,  of  solid  carbon  particles  the 
light  of  our  lamps  is  mainly  due.  But  the  existence  of 
these  particles,  in  the  single  state,  implies  the  absence  of 


62  LECTUKE   H. 

oxygen  to  seize  hold  of  them.  If,  at  the  moment  of  their 
liberation  from  the  hydrogen  with  which  they  are  first 
combined,  oxygen  were  present  to  seize  upon  them,  their 
state  of  bachelorhood  would  be  extinguished,  and  we  should 
no  longer  have  their  light.  Thus,  when  we  mix  a  sufficient 
quantity  of  air  with  the  gas  issuing  from  a  jet,  when  we 
mix  it  so  that  the  oxygen  penetrates  to  the  very  heart  of 
the  jet,  we  fi-nd  the  light  destroyed.  Here  is  a  burner,  in- 
vented by  Prof.  Bunsen,  for  the  express  purpose  of  destroy- 
ing the  light  by  causing  the  quick  combustion  of  the  car- 
bon particles.  The  burner  from  which  the  gas  escapes  is 
introduced  into  a  tube  ;  this  tube  is  perforated  nearly  on  a 
level  with  the  gas  orifice,  and  through  these  perforations 
the  air  enters,  mingles  with  the  gas,  and  the  mixture  issues 
from  the  top  of  the  tube.  Fig.  19  repre- 
sents a  form  of  this  burner  ;  the  gas  is  dis- 
charged into  the  perforated  chamber  a, 
where  air  mingles  with  it,  and  both  ascend 
the  tube  a  b  together  :  d  is  a  rose-burner, 
which  may  be  used  to  vary  the  shape  of 
the  flame.  I  ignite  the  mixture,  but  the 
flame  produces  hardly  any  light.  Heat  is 
the  thing  here  aimed  at,  and  this  lightless 
flame  is  much  hotter  than  the  ordinary 
flame,  because  the  combustion  is  much  quicker,  and  there- 
fore more  intense.*  If  I  stop  the  orifices  in  a  I  cut  off  the 
supply  of  air,  and  the  flame  at  once  becomes  luminous : 
we  have  now  the  ordinary  case  of  a  core  of  unburnt  gas 
surrounded  by  a  burning  shell.  The  illuminating  power  of 
a  gas  may,  in  fact,  be  estimated  by  the  quantity  of  air 
necessary  to  prevent  the  precipitation  of  the  solid  carbon 
particles  ;  the  richer  the  gas,  the  more  air  will  be  required 
to  produce  this  effect. 

An  interesting  observation  may  be  made  on  almost  any 
windy  Saturday  evening  in  the  streets  of  London,  on  the 
*  Not  hotter,  nor  nearly  so  hot,  to  a  body  exposed  to  its  radiation; 
but  very  much  hotter  to  a  body  plunged  in  thejlame. 


COMBUSTION  ON  MONT  BLANC.  63 

sudden,  and  almost  total  extinction  of  the  light  of  the  huge 
gas  jets,  exposed  chiefly  in  butchers'  shops.  When  the 
wind  blows,  the  oxygen  is  carried  mechanically  to  the  very 
heart  of  the  flame,  and  the  white  light  instantly  vanishes 
to  a  pale  and  ghastly  blue.  During  festive  illuminations 
the  same  effect  may  be  observed ;  the  absence  of  the  light 
being  due,  as  in  the  case  of  Bunsen's  burner,  to  the  pres- 
ence of  a  sufficient  amount  of  oxygen  to  consume,  instant- 
ly, the  carbon  of  the  flame. 

To  determine  the  influence  of  height  upon  the  rate  of 
combustion,  was  one  of  the  problems  which  I  had  set  be- 
fore me,  in  my  journey  to  the  Alps  in  1859.  Fortunately 
for  science,  I  invited  Dr.  Frankland  to  accompany  me  on 
the  occasion,  and  to  undertake  the  experiments  on  combus- 
tion, while  I  proposed  devoting  myself  to  observations  on 
solar  radiation.  The  plan  pursued  was  this  :  six  candles 
were  purchased  at  Chamouni  and  carefully  weighed  ;  they 
were  then  allowed  to  burn  for  an  hour  in  the  Hotel  de 
1'IJnion,  and  the  loss  of  weight  was  determined.  The  same 
candles  were  taken  to  the  summit  of  Mont  Blanc,  and  on 
the  morning  of  Aug.  21,  were  allowed  to  burn  for  an  hour 
in  a  tent,  which  perfectly  sheltered  them  from  the  action 
of  the  wind.  The  aspect  of  the  six  flames  at  the  summit 
surprised  us  both.  They  seemed  the  mere  ghosts  of  the 
flames  which  the  same  candles  were  competent  to  produce 
in  the  valley  of  Chamouni — pale,  small,  feeble,  and  sug- 
gesting to  us  a  greatly  diminished  energy  of  combustion. 
The  candles  being  carefully  weighed  on  our  return,  the  un- 
expected fact  was  revealed,  that  the  quantity  of  stearine 
consumed  above  was  almost  precisely  the  same  as  that 
consumed  below.  Thus,  though  the  light-giving  power  of 
the  flame  was  diminished  in  an  extraordinary  degree  by 
the  elevation,  the  energy  of  the  combustion  was  the  same 
above  as  it  was  below.  This  curious  result  is  to  be  ascribed 
mainly  to  the  mobility  of  the  air  at  this  great  height.  The 


64:  LECTURE    n. 

particles  of  oxygen  could  penetrate  the  flame  with  com« 
parative  freedom,  thus  destroying  its  light,  and  making 
atonement  for  the  smallness  of  their  number  by  the  prompt- 
ness of  their  action. 

Dr.  Frankland  has  made  these  experiments  the  basis 
of  a  most  interesting  memoir.*  He  shows  that  the  quan- 
tity of  a  candle  consumed  in  a  given  time  is,  within  wide 
limits,  independent  of  the  density  of  the  air  ;  and  the  rea- 
son is,  that  although  by  compressing  the  air  we  augment 
the  number  of  active  particles  in  contact  with  the  flame, 
we  almost,  in  the  same  degree,  diminish  their  mobility,  and 
retard  their  combustion.  When  an  excess  of  air,  moreover, 
surrounds  the  flame,  its  chilling  effect  will  tend  to  prolong 
the  existence  of  the  carbon  particles  in  a  solid  form,  and 
even  to  prevent  their  final  combustion.  One  of  the  beauti- 
ful experimental  results  of  Dr.  Frankland's  investigation 
is,  that  by  condensing  the  air  around  it,  the  pale  and 
smokeless  flame  of  a  spirit  lamp  may  be  rendered  as  bright 
as  that  of  coal  gas,  and,  by  pushing  the  condensation  suffi- 
ciently far,  the  flame  may  actually  be  rendered  smoky,  the 
sluggish  oxygen  present  being  incompetent  to  effect  the 
complete  combustion  of  the  carbon. 

But  to  return  to  our  theory  of  combustion :  it  is  to  the 
clashing  together  of  the  oxygen  of  the  air  and  the  constitu- 
ents of  our  gas  and  candles,  that  the  light  and  heat  of 
our  flames  are  due.  I  scatter  steel  filings  in  this  flame,  and 
you  see  the  star-like  scintillations  produced  by  the  com- 
bustion of  the  steel.  Here  the  steel  is  first  heated,  till  the 
attraction  between  it  and  the  oxygen  becomes  sufficiently 
strong  to  cause  them  to  combine,  and  these  rocket-like 
flashes  are  the  result  of  their  collision.  It  is  the  impact 
of  the  atoms  of  oxygen  against  the  atoms  of  sulphur  which 
produces  the  flame  observed  when  sulphur  is  burned  in 

*  Philosophical  Transactions  for  1861. 


CLASHING  OF  ATOMS.  65 

oxygen  or  in  air ;  to  the  collision  of  the  same  atoms 
against  phosphorus  are  due  the  intense  heat  and  dazzling 
light  which  result  from  the  combustion  of  phosphorus  in 
oxygen  gas.  It  is  the  collision  of  chlorine  and  antimony 
which  produces  the  light  and  heat  observed  where  these 
bodies  are  mixed  together ;  and  it  is  the  clashing  of  sul- 
phur and  copper  which  causes  the  incandescence  of  the 
mass  when  these  substances  are  heated  together  in'  a  Flo- 
rence flask.  In  short,  all  cases  of  combustion  are  to  be  as- 
cribed to  the  collision  of  atoms  which  have  been  urged  to- 
gether by  their  mutual  attractions. 

NOTES. 

(1)  A  far  more  beautiful  mode  of  demonstration  was  subsequently  re- 
sorted to.     Removing  the  lens  from  the  camera  of  the  electric  lamp,  the 
rays  from  the  coal-points  issued  divergent.     I  placed  a  large  plano-convex 
lens  in  front,  so  as  to  convert  the  divergent  cone  into  a  convergent  one, 
and  caused  the  cone  to  pass  through  the  receiver.     Its  track  was  at  first 
invisible,  but  two  or  three  strokes  of  the  pump  precipitated  the  vapour, 
and  then  the  track  of  the  beam  resembled  a  white  solid  bar.     After  cross- 
ing the  receiver,  the  light  fell  upon  a  white  screen,  and  exhibited  splendid 
diffraction  colours  when  the  cloud  formed. 

(2)  Liebig's  Annalen,  vol.  xlii.  p.  233 ;  Phil.  Mag.  4th  Series,  vol.  xxiv. 
p.  371 ;  and  in  resume,  Phil.  Mag.  vol.  xxv.  p.  378.     I  am  indebted  to  Mr. 
Wheatstone  for  the  perusal  of  a  rare  and  curious  pamphlet  by  G.  Reben- 
stein,  with  the  following  (translated)  title :  '  Progress  of  our  Time.     Gen- 
eration of  Heat  without  Fuel ;  or,  Description  of  a  Mechanical  Process, 
based  on  physical  and  mathematical  proofs,  by  which  Caloric  may  be  ex- 
tracted from  Atmospheric  Air,  and  in  a  high  degree  concentrated.     The 
cheapest  Substitute  for  Fuel  in  most  cases  where  combustion  is  necessary.' 
Rebenstein  deduces  from  the  experiments  of  Dulong  the  quantity  of  heat 
evolved  in  the  compression  of  a  gas.     No  glimpse  of  the  dynamical  theory 
is,  however,  to  be  found  in  his  paper ;  his  heat  is  matter  ( Warmestojf) 
which  is  squeezed  out  of  the  air  as  water  is  out  of  a  sponge. 

(3)  In  1843  an  essay  entitled  'Theses  concerning  Force'  was  pre- 
sented to  the  Royal  Society  of  Copenhagen  by  a  Danish  philosopher 
named  Colding.     At  this  early  date  M.  Colding  sought  to  ascertain  the 
quantity  of  heat  generated  by  the  friction  of  various  metals  against  each 


66  LECTURE   II. 

other  and  against  other  substances,  and  to  determine  the  amount  of  me- 
chanical work  consumed  in  its  generation.  In  an  account  of  his  researches 
given  by  himself  in  the  Philosophical  Magazine  (vol.  xxvii.  p.  56),  he 
states  that  the  result  of  his  experiments,  nearly  200  in  number,  was  that 
the  heat  disengaged  was  always  in  proportion  to  the  mechanical  energy 
lost.  Independently  of  the  materials  by  which  the  heat  was  generated, 
M.  Colding  found  that  an  amount  of  heat  competent  to  raise  a  pound  of 
water  1°  C.  would  raise  a  weight  of  a  pound  1148  feet  high.  M.  Colding 
starts  from  the  principle  that  '  as  the  forces  of  nature  are  something  spir- 
itual and  immaterial — entities  whereof  we  are  cognisant  only  by  their  mas- 
tery over  nature,  those  entities  must  of  course  be  very  superior  to  every- 
thing material  in  the  world ;  and  as  it  is  obvious  that  it  is  through  them 
only  that  the  wisdom  we  perceive  and  admire  in  nature  expresses  itself, 
these  powers  must  evidently  be  in  relation  to  the  spiritual,  immaterial,  and 
intellectual  power  itself  that  guides  nature  in  its  progress ;  but  if  such  is 
the  case  it  is  consequently  quite  impossible  to  conceive  of  these  forces  as 
anything  naturally  mortal  or  perishable.  Surely,  therefore,  the  forces 
ought  to  be  regarded  as  absolutely  imperishable.'  Whatever  induces  a 
man  to  work  has  some  value ;  and  inasmuch  as  these  speculations  induced 
M.  Colding  to  become  an  experimenter,  they  are  on  this  account  entitled 
to  a  certain  degree  of  respect. 


APPENDIX   TO  LECTURE  II. 


EXTRACTS  FROM  THE  TWENTIETH  APHORISM  OF  THE  SECOND 
BOOK  OF  THE  'NOVUM  OEGANUM.' 


I  say  of  motion  that  it  is  the  genus  of  which  heat  is  a 
species,  I  would  be  understood  to  mean,  not  that  heat  generates 
motion,  or  that  motion  generates  heat  (though  both  are  true  in 
certain  cases),  but  that  heat  itself,  its  essence  and  quiddity,  is 
motion,  and  nothing  else  ;  limited,  however,  by  the  specific  differ- 
ences which  I  will  presently  subjoin,  as  soon  as  I  have  added  a 
few  cautions,  for  the  sake  of  avoiding  ambiguity.  .  .  . 

Nor,  again,  must  the  communication  of  heat,  or  its  transitive 
nature,  by  means  of  which  a  body  becomes  hot  when  a  hot  body 
is  applied  to  it,  be  confounded  with  the  form  of  heat.  For  heat 
is  one  thing,  and  heating  is  another.  Heat  is  produced  by  the 
motion  of  attrition  without  any  preceding  heat.  .  .  . 

Heat  is  an  expansive  motion,  whereby  a  body  strives  to  dilate 
and  stretch  itself  to  a  larger  sphere  or  dimension  than  it  had  pre- 
viously occupied.  This  difference  is  most  observable  in  flame, 
where  the  smoke  or  thick  vapour  manifestly  dilates  and  expands 
into  flame. 

It  is  shown  also  in  all  boiling  liquid,  which  manifestly  swells, 
rises,  and  bubbles,  and  carries  on  the  process  of  self-expansion, 
till  it  turns  into  a  body  far  more  extended  and  dilated  than  the 

liquid  itself,  namely,  into  vapour,  smoke,  or  air. 

******* 

The  third  specific  difference  is  this,  that  heat  is  a  motion  of 
expansion,  not  uniformly  of  the  whole  body  together,  but  in  the 
smaller  parts  of  it  ;  and  at  the  same  time  checked,  repelled,  and 
beaten  back,  so  that  the  body  acquires  a  motion  alternative,  per- 


68  APPENDIX  TO  LECTURE  H. 

petually  quivering,  striving   and  struggling,  and  irritated  by  re- 
percussion, whence  springs  the  fury  of  fire  and  heat. 

Again,  it  is  shown  in  this  that  when  the  air  is  expanded  in  a 
calender  glass,  without  impediment  or  repulsion,  that  is  to  say, 
uniformly  and  equably,  there  is  no  perceptible  heat.  Also,  when 
wind  escapes  from  confinement,  although  it  bursts  forth  with  the 
greatest  violence,  there  is  no  very  great  heat  perceptible ;  because 
the  motion  is  of  the  whole,  without  a  motion  alternating  in  the 
particles. 

And  this  specific  difference  is  common  also  to  the  nature  of 
cold ;  for  in  cold  contractive  motion  is  checked  by  a  resisting 
tendency  to  expand,  just  as  in  heat  the  expansive  action  is  checked 
by  a  resisting  tendency  to  contract.  Thus  whether  the  particles 
of  a  body  work  inward  or  outward,  the  mode  of  action  is  the 
same. 

******* 

Now  from  this  our  first  vintage  it  follows,  that  the  form  or 
true  definition  of  heat  (heat  that  is  in  relation  to  the  universe,  not 
simply  in  relation  to  man)  is  in  a  few  words  as  follows :  Heat  is 
a  motion,  expansive,  restrained,  and  acting  in  its  strife  upon  the 
smaller  particles  of  lodies^  But  the  expansion  is  thus  modified : 
while  it  expands  all  ways,  it  lias  at  the  same  time  an  inclination  up- 
wards. And  the  struggle  in  the  particles  is  modified  also ;  it  is 
not  sluggish,  Tjut  hurried  and  with  violence* 


ABSTRACT  OF  COUNT  EUMFOED'S  ESSAY,  ENTITLED  'AN  ENQUIRY 
CONCERNING  THE  SOURCE  OF  THE  HEAT  WHICH  IS  EXCITED  BY 
FRICTION.' 

[Read  before  the,  Eoyal  Society,  January  25,  1798.] 

Being  engaged  in  superintending  the  boring  of  cannon  in  the 
workshops  of  the  military  arsenal  at  Munich,  Count  Rumford  was 
struck  with  the  very  considerable  degree  of  heat  which  a  brass 
gun  acquires,  in  a  short  time,  in  being  bored,  and  with  the  still 
more  intense  heat  (much  greater  than  that  of  boiling  water)  of 

*  Bacon's  "Works,  vol.  iv. :  Spedding's  Translation. 


EUMFOKD'S  EXPEEIMENTS.  C9 

the  metallic  chips  separated  from  it  by  the  borer,  he  proposed  to 
himself  the  following  questions  : 

4  Whence  comes  the  heat  actually  produced  in  the  mechanical 
operation  above  mentioned  ? 

'  Is  it  furnished  by  the  metallic  chips  which  are  separated  from 
the  metal  ? ' 

If  this  were  the  case,  then  the  capacity  for  heat  of  the  parts  of 
the  metal  so  reduced  to  chips  ought  not  only  to  be  changed,  but 
the  change  undergone  by  them  should  be  sufficiently  great  to  ac- 
count for  all  the  heat  produced.  No  such  change,  however,  had 
taken  place  ;  for  the  chips  were  found  to  have  the  same  capacity 
as  slices  of  the  same  metal  cut  by  a  fine  saw,  where  heating  was 
avoided.  Hence,  it  is  evident  that  the  heat  produced  could  not 
possibly  have  been  furnished  at  the  expense  of  the  latent  heat  of 
the  metallic  chips.  Kumford  describes  those  experiments  at  length, 
and  they  are  conclusive. 

He  then  designed  a  cylinder  for  the  express  purpose  of  gener- 
ating heat  by  friction,  by  having  a  blunt  borer  forced  against  its 
solid  bottom,  while  the  cylinder  was  turned  round  its  axis  by  the 
force  of  horses.  To  measure  the  heat  developed,  a  small  round 
hole  was  bored  in  the  cylinder  for  the  purpose  of  introducing  a 
small  mercurial  thermometer.  The  weight  of  the  cylinder  was 
113.13  Ibs.  avoirdupois. 

The  borer  was  a  flat  piece  of  hardened  steel,  0'63  of  an  inch 
thick,  4  inches  long,  and  nearly  as  wide  as  the  cavity  of  the  bore 
of  the  cylinder,  namely,  3|  inches.  The  area  of  the  surface  by 
which  its  end  was  in  contact  with  the  bottom  of  the  bore  was 
nearly  2^  inches.  At  the  beginning  of  the  experiment  the  tem- 
perature of  the  air  in  the  shade  and  also  that  of  the  cylinder 
was  60  degrees  Fahr.  At  the  end  of  30  minutes,  and  after  the 
cylinder  had  made  960  revolutions  round  its  axis,  the  temperature 
was  found  to  be  130  degrees. 

Having  taken  away  the  borer,  he  now  removed  the  metallic 
dust,  or  rather  scaly  matter,  which  had  been  detached  from  the 
bottom  of  the  cylinder  by  the  blunt  steel  borer,  and  found  its 
weight  to  be  837  grains  troy.  '  Is  it  possible,'  he  exclaims,  *  that 
the  very  considerable  quantity  of  heat  produced  in  this  experi- 
ment— a  quantity  which  actually  raised  the  temperature  of  above 
113  pounds  of  gun  metal  at  least  70  degrees  of  Fahrenheit's  ther- 


YO  APPENDIX  TO  LECTTJUE  H. 

mometer — could  have  been  furnished  by  so  inconsiderable  a  quan- 
tity of  metallic  dust,  and  this  merely  in  consequence  of  a  change 
in  its  capacity  for  heat  ? 

*  But  without  insisting  on  the  improbability  of  this  supposi- 
tion, we  have  only  to  recollect  that  from  the  results  of  actual  and 
decisive  experiments,  made  for  the  express  purpose  of  ascertaining 
that  fact,  the  capacity  for  heat  of  the  metal  of  which  great  guns 
are  cast  is  not  sensibly  changed  by  being  reduced  to  the  form  of 
metallic  chips,  and  there  does  not  seem  to  be  any  reason  to  think 
that  it  can  be  much  changed,  if  it  be  changed  at  all,  in  being  re- 
duced to  much  smaller  pieces  by  a  borer  which  is  less  sharp.' 

He  next  surrounded  his  cylinder  by  an  oblong  deal  box,  in 
such  a  manner  that  the  cylinder  could  turn  water-tight  in  the  cen- 
tre of  the  box,  while  the  borer  was  pressed  against  the  bottom  of 
the  cylinder.  The  box  was  filled  with  water  until  the  entire  cyl- 
inder was  covered,  and  then  the  apparatus  was  set  in  action. 
The  temperature  of  the  water  on  commencing  was  60  degrees. 

'  The  result  of  this  beautiful  experiment,'  writes  Rumford, 
'  was  very  striking,  and  the  pleasure  it  afforded  me  amply  repaid 
me  for  all  the  trouble  I  had  had  in  contriving  and  arranging  the 
complicated  machinery  used  in  making  it.  The  cylinder  had 
been  in  motion  but  a  short  time,  when  I  perceived,  by  putting  my 
hand  into  the  water,  and  touching  the  outside  of  the  cylinder, 
that  heat  was  generated. 

*  At  the  end  of  an  hour  the  fluid,  which  weighed  18'77  Ibs.,  or 
2£  gallons,  had  its  temperature  raised  47  degrees,  being  now  107 
degrees. 

'  In  thirty  minutes  more,  or  one  hour  and  thirty  minutes  after 
the  machinery  had  been  set  in  motion,  the  heat  of  the  water  was 
142  degrees. 

*  At  the  end  of  two  hours  from  the  beginning,  the  temperature 
was  178  degrees. 

*  At  two  hours  and  twenty  minutes  it  was  200  degrees,  and  at 
two  hours  and  thirty  minutes  it  ACTUALLY  BOILED  ! ' 

It  is  in  reference  to  this  experiment  that  Rumford  made  the 
remarks  regarding  the  surprise  of  the  bystanders,  which  I  have 
quoted  in  Lecture  I. 

He  then  carefully  estimates  the  quantity  of  heat  possessed  by 
each  portion  of  his  apparatus  at  the  conclusion  of  the  experiment, 


EXPEEIM 


and  adding  all  together,  finds  a  total  sufficient  to  raise  26'58  Ibs. 
of  ice-cold  water  to  its  boiling  point,  or  through  ISO  degrees  Fah- 
renheit. By  careful  calculation,  he  finds  this  heat  equal  to  that 
given  out  by  the  combustion  of  2303-8  grains  (=  4^  oz.  troy) 
of  wax. 

He  then  determines  the  *  celerity '  with  which  the  heat  was 
generated,  summing  up  his  computations  thus  :  '  From  the  results 
of  these  computations,  it  appears  that  the  quantity  of  heat  pro- 
duced equably,  or  in  a  continuous  stream,  if  I  may  use  the  expres- 
sion, by  the  friction  of  the  blunt  steel  borer  against  the  bottom 
of  the  hollow  metallic  cylinder,  was  greater  than  that  produced  in 
the  combustion  of  nine  wax  candles,  each  £  of  an  inch  in  diameter, 
all  burning  together  with  clear  bright  flames.' 

'  One  horse  would  have  been  equal  to  the  work  performed, 
though  two  were  actually  employed.  Heat  may  thus  be  produced 
merely  by  the  strength  of  a  horse,  and,  in  a  case  of  necessity,  this 
heat  might  be  used  in  cooking  victuals.  But  no  circumstances 
could  be  imagined  in  which  this  method  of  procuring  heat  would 
be  advantageous  ;  for  more  heat  might  be  obtained  by  using  the 
fodder  necessary  for  the  support  of  a  horse  as  fuel.' 

[This  is  an  extremely  significant  passage,  intimating  as  it 
does,  that  Rumford  saw  clearly  that  the  force  of  animals  was  de- 
rived from  the  food  ;  no  creation  of  force  taking  place  in  the  ani- 
mal body.] 

*  By  meditating  on  the  results  of  all  these  experiments  we  are 
naturally  brought  to  that  great  question  which  has  so  often  been 
the  subject  of  speculation  among  philosophers,  namely,  What  is 
heat — is  there  any  such  thing  as  an  igneous  fluid  ?  Is  there  any 
thing  that,  with  propriety,  can  be  called  caloric  ?  ' 

'  We  have  seen  that  a  very  considerable  quantity  of  heat  may 
be  excited  by  the  friction  of  two  metallic  surfaces,  and  given  off 
in  a  constant  stream  or  flux  in  all  directions,  without  interruption 
or  intermission,  and  without  any  signs  of  diminution  or  exhaustion. 
In  reasoning  on  this  subject  we  must  not  forget  that  most  remark- 
able circumstance,  that  the  source  of  the  heat  generated  by  friction 
in  thr.3e  experiments  appeared  evidently  to  be  inexhaustible.  (The 
italics  are  Rumford's.)  It  is  hardly  necessary  to  add,  that  any- 
thing which  any  insulated  body  or  system  of  bodies  can  continue 
to  furnish  without  limitation  cannot  possibly  be  a  material  sub- 


72  APPENDIX  TO   LECTTJKE  II. 

stance;  and  it  appears  to  me  to  be  extremely  difficult,  if  not 
quite  impossible,  to  form  any  distinct  idea  of  anything  capable 
of  being  excited  and  communicated  in  those  experiments,  except 
it  be  MOTION. 

When  the  history  of  the  dynamical  theory  of  lieat  is  written, 
the  man  who,  in  opposition  to  the  scientific  belief  of  his  time, 
could  experiment  and  reason  upon  experiment,  as  Rumford  did  in 
the  investigation  here  referred  to,  cannot  be  lightly  passed  over. 
Hardly  anything  more  powerful  against  the  materiality  of  heat 
has  been  since  adduced,  hardly  anything  more  conclusive  in  the 
way  of  establishing  that  heat  is  what  Rumford  considered  it  to 
be,  Motion.  $ 


ON  THE  COMPEESSION  OF  AIR  CONTAINING  BISULPHIDE 
OF  CAPJBON  VAPOUE. 

'  A  very  singular  phenomenon  was  repeatedly  observed  during 
the  experiments  with  bisulphide  of  carbon.  After  determining 
the  absorption  of  the  vapour,  the  tube  was  exhausted  as  perfectly 
as  possible,  the  trace  of  vapour  left  behind  being  exceedingly 
minute.  Dry  air  was  then  admitted  to  cleanse  the  tube.  On 
again  exhausting,  after  the  first  few  strokes  of  the  pump,  a  jar 
was  felt  and  a  kind  of  explosion  heard,  while  dense  volumes  of 
blue  smoke  immediately  issued  from  the  pump  cylinders.  The 
action  was  confined  to  the  latter,  and  never  propagated  itself 
backwards  into  the  experimental  tube. 

'  Ikis  only  with  bisulphide  of  carbon  that  this  effect  has  been 
observed.  It  may,  I  think,  be  explained  in  the  following  man- 
ner : — To  open  the  valve  of  the  piston,  the  gas  beneath  it  must 
have  a  certain  tension,  and  the  compression  necessary  to  produce 
this  appears  sufficient  to  cause  the  combination  of  the  constituents 
of  the  bisulphide  of  carbon  with  the  oxygen  of  the  air.  Such  a 
combination  certainly  takes  place,  for  the  odour  of  sulphurous 
acid  is  unmistakeable  amid  the  fumes. 

'  To  test  this  idea  I  tried  the  effect  of  compression  in  the  air 
syringe.  A  bit  of  tow  or  cotton  wool  moistened  with  bisulphide 
of  carbon,  and  placed  in  the  syringe,  emitted  a  bright  flash  when 


COMPRESSION  OF  BISULPHIDE  OF  CARBON  VAPOUR.       73 

tlie  air  was  compressed.  By  blowing  out  the  fumes  with  a  glass 
tube,  this  experiment  may  be  repeated  twenty  times  with  the  same 
bit  of  cotton. 

'  It  is  not  necessary  even  to  let  the  moistened  cotton  remain  in 
the  syringe.  -If  the  bit  of  tow  or  cotton  be  thrown  into  it,  and 
out  again  as  quickly  as  it  can  be  ejected,  on  compressing  the  air 
the  luminous  flash  is  seen.  Pure  oxygen  produces  a  brighter  flash 
than  atmospheric  air.  These  facts  are  in  harmony  with  the  above 
explanation.'  * 

*  Phil.  Trans.,  1861 ;  Phil.  Mag.,  Sept.  1861. 


LECTURE    III. 

[February  6,  1862.] 

EXPANSION  :  THK  SOLID,  LIQUID,  AND  GASEOUS  FORMS  OF  MATTER — HYPO- 
THESES REGARDING  THE  CONSTITUTION  OF  GASES — COEFFICIENT  OF  EX- 
PANSION  HEAT  IMPARTED  TO  A  GAS  UNDER  CONSTANT  PRESSURE HEAT 

IMPARTED    TO   A    GAS   AT    CONSTANT    VOLUME MAYER'S    CALCULATION    OF 

THE  MECHANICAL  EQUIVALENT  OF  HEAT — DILATATION  OJ1  GASES  WITHOUT 
REFRIGERATION — ABSOLUTE  ZERO  OF  TEMPERATURE — EXPANSION  OF  LI- 
QUIDS AND  SOLIDS  :  ANOMALOUS  DEPORTMENT  OF  WATER  AND  BISMUTH 
— ENERGY  OF  THE  FORCE  OF  CRYSTALLIZATION — THERMAL  EFFECT  OF 

STRETCHING  WIRES — ANOMALOUS  DEPORTMENT  OF  INDIA-RUBBER. 

APPENDIX: — ADDITIONAL  DATA  CONCERNING  EXPANSION — EXTRACTS  FROM 
SIR  H.  DAVY'S  FIRST  SCIENTIFIC  MEMOIR  :  FUSION  OF  ICE  BY  FRICTION,  &c. 

YOUR,  reappearance  here  to-day,  after  the  strain  which 
has  already  been  put  upon  your  attention,  encourages 
me  to  hope  that  our  present  experiment  will  not  be  entirely 
unsuccessful.  I  need  not  tell  an  audience  like  this  that 
nothing  intellectually  great  is  either  accomplished  or  ap- 
propriated without  effort.  Newton  ascribed  the  difference 
between  himself  and  other  men  to  his  patience  in  steadily 
looking  at  a  question,  until  light  dawned  upon  it,  and  if  we 
have  firmness  to  imitate  his  example,  we  shall,  no  doubt, 
reap  a  commensurate  reward. 

In  our  first  lecture  I  permitted  a  sledge-hammer  to  de- 
scend upon  a  mass  of  lead,  and  we  found  that  the  lead  be- 
came heated,  as  soon  as  the  mechanical  motion  of  the  ham- 
mer was  arrested.  Formerly  it  was  assumed  that  the  force 


EXPANSION.  75 

of  the  hammer  was  simply  lost  by  the  concussion.  In  elas« 
tic  bodies  it  was  supposed  that  a  portion  of  the  force  was 
restored  by  the  elasticity  of  the  body,  which  caused  the 
descending  mass  to  rebound ;  but  in  the  collision  of  inelas- 
tic bodies  it  was  taken  for  granted  that  the  force  of  impact 
was  lost.  This,  according  to  our  present  notions,  was  a 
fundamental  mistake  ;  we  now  admit  no  loss,  but  assume, 
that  when  the  motion  of  the  descending  hammer  ceases,  it 
is  simply  a  case  of  transference,  instead  of  annihilation. 
The  motion  of  the  mass,  as  a  whole,  has  been  transformed 
into  a  motion  of  the  molecules  of  the  mass.  This  motion 
of  heat,  however,  though  intense,  is  executed  within  limits 
too  minute,  and  the  moving  particles  are  too  small,  to  be 
visible.  To  discern  these  processes  we  must  make  use  of 
a  finer  eye  and  higher  powers,  namely,  the  eye  and  powers 
of  the  mind.  In  the  case  of  solid  bodies,  then,  while  the 
force  of  cohesion  still  holds  the  particles  together,  you 
must  conceive  a  power  of  vibration,  within  certain  limits, 
to  be  possessed  by  the  particles.  You  must  suppose  them 
oscillating  to  and  fro  across  their  positions  of  rest ;  and 
the  greater  the  amount  of  heat  we  impart  to  the  body,  or 
the  greater  the  amount  of  mechanical  action  which  we  in- 
vest in  it  by  percussion,  compression,  or  friction,  the  more 
intense  will  be  the  molecular  vibration,  and  the  wider  the 
amplitude  of  the  atomic  oscillations. 

Now,  nothing  is  more  natural  than  that  particles  thus 
vibrating,  and  ever  as  it  were  seeking  wider  room,  should 
urge  each  other  apart,  and  thus  cause  the  body  of  which 
they  are  the  constituents,  to  expand  in  volume.  This,  in 
general,  is  the  consequence  of  imparting  heat  to  bodies — 
expansion  of  volume.  We  shall  closely  consider  the  few 
apparent  exceptions  to  this  law  by  and  by.  By  the  force 
of  cohesion,  then,  the  particles  are  held  together ;  by  the 
force  of  heat  they  are  pushed  asunder :  here  are  the  two 
antagonist  principles  on  which  the  molecular  aggregation 


70  LECTUKE  m. 

of  the  body  depends.  Let  us  suppose  the  communication 
of  heat  to  continue ;  every  increment  of  heat  pushes  the 
particles  more  widely  apart ;  but  the  force  of  cohesion,  like 
all  other  known  forces,  acts  more  and  more  feebly,  as  the 
distance  between  the  particles  which  are  the  seat  of  the 
force  is  augmented.  As,  therefore,  the  heat  strengthens, 
its  opponent  grows  weak,  until,  finally,  the  particles  are  so 
far  loosed  from  the  rigid  thrall  of  cohesion,  that  they  are 
at  liberty,  not  only  to  vibrate  to  and  fro  across  a  fixed  po- 
sition, but  also  to  roll  or  glide  around  each  other.  Cohesion 
is  not  yet  destroyed,  but  it  is  so  far  modified,  that  while  the 
particles  still  offer  resistance  to  being  torn  directly  asunder, 
their  lateral  mobility  over  each  other's  surfaces  is  secured. 
This  is  the  liquid  condition  of  matter. 

In  the  interior  of  a  mass  of  liquid  the  motion  of  every 
atom  is  controlled  by  the  atoms  which  surround  it.  But 
suppose  you  develope  heat  of  sufficient  power  within  the 
body  of  a  liquid,  what  occurs  ?  Why,  the  particles  break 
the  last  fetters  of  cohesion,  and  fly  asunder  to  form  bubbles 
of  vapour.  If  one  of  the  surfaces  of  the  liquid  be  quite 
free,  that  is  to  say,  uncontrolled  either  by  a  liquid  or  solid ; 
it  is  quite  easy  to  conceive  that  some  of  the  vibrating  su- 
perficial particles  will  be  jerked  quite  away  from  the  liquid, 
and  will  fly  with  a  certain  velocity  through  space.  Thus 
freed  from  the  influence  of  cohesion,  we  have  matter  in  the 
vaporous  or  gaseous  form. 

My  object  here  is  to  familiarize  your  minds  with  the 
general  conception  of  atomic  motion.  I  have  spoken  of  the 
vibration  of  the  particles  of  a  solid  as  causing  its  expan- 
sion ;  the  particles  have  been  thought  by  some  to  revolve 
round  each  other,  and  the  communication  of  heat,  by  aug- 
menting the  centrifugal  force  of  the  particles,  was  supposed 
to  push  them  more  widely  asunder.  I  have  here  a  weight 
attached  to  a  spiral  spring  ;  if  I  twirl  the  weight  round  in 
the  air  it  tends  to  fly  away  from  me,  the  spring  stretches  to 


ATOMIC  PROJECTILES. 

a  certain  extent,  and  as  I  augment  the  speed  of  revolution, 
the  spring  stretches  still  more,  the  distance  between  my 
hand  and  the  weight  being  thus  augmented.  It  has  been 
thought  that  the  augmentation  of  the  distance  between  a 
body's  atoms  by  heat,  may  be  also  due  to  a  revolution  of 
its  particles.  And  imagine  the  motion  to  continue  till  the 
spring  snaps  ;  the  ball  attached  to  it  would  fly  off  along  a 
tangent  to  its  former  orbit,  and  thus  represent  an  atom 
freed,  by  heat,  from  the  force  of  cohesion,  which  is  rudely 
represented  by  our  spring.  The  ideas  of  the  most  well- 
informed  philosophers  are  as  yet  uncertain  regarding  the 
exact  nature  of  the  motion  of  heat ;  but  the  great  point, 
at  present,  is  to  regard  it  as  motion  of  some  kind,  leaving 
its  more  precise  character  to  be  dealt  with  in  future  inves- 
tigations. 

We  might  extend  the  notion  of  revolving  atoms  to 
gases  also,  and  deduce  their  phenomena  from  a  motion  of 
this  kind.  But  I  have  just  thrown  out  an  idea  regarding 
gaseous  particles,  which  is  at  present  very  ably  main- 
tained :  *  the  idea,  namely,  that  such  particles  fly  in 
straight  lines  through  space.  Everybody  must  have  re- 
marked how  quickly  the  perfume  of  an  odorous  body  fills 
a  room,  and  this  fact  harmonizes  with  the  idea  of  the  di- 
rect projection  of  the  particles.  But  it  may  be  proved, 
that  if  the  theory  of  rectilinear  motion  be  'true,  the  parti- 
cles must  move  at  the  rate  of  several  hundred  feet  a  sec- 
ond. Hence  it  might  be  objected  that,  according  to  the 
above  hypothesis,  odours  ought  to  spread  much  more 
quickly  than  they  are  observed  to  do. 

The  answer  to  this  objection  is,  that  they  have  to  make 
their  way  through  a  crowd  of  air  particles,  with  which 
they  come  into  incessant  collision.  On  an  average,  the 

*  By  Joule,  Krb'nig,  Maxwell ;  and,  in  a  series  of  extremely  able  papers, 
by  Clausius. 


78  LECTUEE  m. 

distance  through  which  an  odorous  particle  can  travel  la. 
common  air,  without  striking  against  a  particle  of  air,  is 
infinitesimal,  and  hence  the  propagation  of  a  perfume 
through  air  is  enormously  retarded  by  the  air  itself.  It  is 
well  known  that  when  a  free  communication  is  opened  be- 
tween the  surface  of  a  liquid  and  a  vacuum,  the  vacuous 
space  is  much  more  speedily  filled  to  saturation  with  the 
vapour  of  the  liquid,  than  when  air  is  present.* 

According  to  this  hypothesis,  then,  we  are  to  figure  a 
gaseous  body  as  one  whose  particles  are  flying  in  straight 
lines  through  space,  impinging  like  little  projectiles  upon 
each  other,  and  striking  against  the  boundaries  of  the  space 
which  they  occupy.  Mr.  Anderson  will  place  this  bladder, 
half  filled  with  air,  under  the  receiver  of  the  air-pump  ;  he 
will  now  work  the  pump,  and  remove  the  air  that  surrounds 
the  bladder.  The  bladder  swells  ;  the  air  within  it  appears 
quite  to  fill  it,  so  as  to  remove  all  its  folds  and  creases. 
How  is  this  expansion  of  the  bladder  produced  ?  Accord- 
ing to  our  present  theory,  it  is  produced  by  the  shooting 
of  atomic  projectiles  against  its  interior  surface,  which 
drive  the  envelope  outwards,  until  its  tension  is  able  to 
cope  with  their  force.  When  air  is  admitted  into  the  re- 
ceiver, the  bladder  shrivels  up  to  its  former  size  ;  and  here 
we  must  figure  the  discharge  of  the  air  particles  against  the 
outer  surface  of  the  bladder,  which  drive  the  envelope  in- 
wards, causing,  at  the  same  time,  the  particles  within  to 
concentrate  their  fire,  until  finally  the  force  from  within 
equals  that  from  without,  and  the  envelope  remains  quies- 
cent. All  the  impressions,  then,  which  we  derive  from 
heated  air  or  vapour  are,  according  to  this  hypothesis, 
due  to  the  impact  of  the  gaseous  atoms.  They  stir  the 
nerves  in  their  own  peculiar  way,  the  nerves  transmit 
the  motion  to  the  brain,  and  the  brain  declares  it  to  be 
heat.  Thus  the  impression  one  receives  on  entering  the 
hot  room  of  a  Turkish  bath,  is  caused  by  the  atomic  can- 
*  See  Note  (4)  at  the  end  of  this  Lecture. 


EXPANSION  OF  GASES  BY  HEAT.  79 

nonade  which  is  there  maintained  agains  the  surface  of 
the  body. 

If,  instead  of  placing  this  bladder  under  the  receiver  of 
an  air-pump,  and  withdrawing  the  external  air,  I  augment, 
by  heat,  the  projectile  force  of  the  particles  within  it,  these 
particles,  though  comparatively  few  in  number,  will  strike 
with  such  impetuous  energy  against  the  inner  surface  as  to 
cause  the  envelope  to  retreat :  the  bladder  swells  and  be- 
comes apparently  filled  with  air ;  I  hold  the  bladder  close 
to  the  fire,  and  here  it  is,  you  see,  with  all  its  creases  re- 
moved. But  you  will  retort,  perhaps,  by  saying  that  this 
ought  not  to  be  the  case,  inasmuch  as  the  air  outside  the 
bladder  is  also  near  the  fire,  and  therefore  animated  with  a 
like  projectile  energy,  which  tends  to  drive  the  envelope 
in.  True,  the  bladder  and  the  air  in  contact  with  it  are 
equally  near  the  fire  ;  but  in  a  future  lecture  you  will  learn 
that  the  air  outside  the  bladder  allows  the  rays  of  heat  to 
pass  through  it  with  very  little  augmentation  of  tempera- 
ture, while  the  bladder  intercepts  the  radiant  heat ;  the  en- 
velope becomes  first  warmed  and  then  communicates  its  heat, 
by  contact,  to  the  air  within.  The  air,  moreover,  in  con- 
tact with  the  bladder  on  the  outside,  though  heated  by  the 
bladder,  has  free  space  to  dilate  in,  and  is  therefore  incom- 
petent to  resist  the  expansion  of  the  confined  air  which  the 
bladder  contains. 

This,  then,  is  a  simple  illustration  of  the  expansive  force 
of  heat,  and  I  have  here  an  apparatus  intended  to  show  you 
the  same  fact  in  another  manner.  Here  is  a  flask,  F  (fig. 
20),  empty,  except  as  regards  air,  which  I  intend  to  heat 
by  this  little  spirit-lamp  underneath.  From  the  flask  a  bent 
tube  passes  to  this  dish,  containing  a  coloured  liquid.  In 
the  dish,  a  2-foot  glass  tube,  1 1,  is  inverted,  closed  at  the 
top,  but  with  its  open  end  downwards ;  you  know  that  the 
pressure  of  the  atmosphere  is  competent  to  keep  the  column 
of  liquid  in  this  tube,  and  here  you  have  it  quite  filled  to 


80 


LECTURE   in. 


the  top  with  the  liquid.  The  tube  passing  from  the  flask 
is  caused  to  turn  up  exactly  underneath  the  open  end  of 
this  upright  tube,  so  that  if  a  bubble  of  air  should  issue 
from  the  former,  it  will  ascend  the  latter.  I  now  heat  the 


Fig.  20. 


flask,  and  as  I  do  so,  the  air  expands,  for  the  reasons  al- 
ready given  ;  bubbles  are  driven  from  the  end  of  the  bent 
tube,  and  they  ascend  in  the  tube  1 1.  The  air  speedily  de- 
presses the  liquid  column,  until  now,  in  the  course  of  a  very 
few  seconds,  the  whole  column  of  liquid  has  been  super- 
seded by  air. 

It  is  perfectly  manifest  that  the  air,  thus  expanded  by 
heat,  is  lighter  than  the  unexpanded  air.  Our  flask,  at  the 
conclusion  of  this  experiment,  is  lighter  than  it  was  at  the 
commencement,  by  the  weight  of  the  air  transferred  from 
it  into  the  upright  tube.  Supposing,  therefore,  a  light  bag 
to  be  filled  with  such  air,  it  is  plain  that  the  bag  would, 
with  reference  to  the  heavy  air  outside  it,  be  like  a  drop 
of  oil  in  water ;  the  oil  being  lighter  than  the  water,  will 


EXPANSION   OF   GASES   BY   HEAT.  81 

ascend  through  the  latter;  so  also  our  bag,  filled  with 
heated  air,  will  ascend  in  the  atmosphere ;  and  this  is  the 
principle  of  the  so-called  fire-balloon.  Mr.  Anderson  will 
ignite  some  tow  in  this  vessel,  over  it  he  will  place  this 
funnel,  and  over  the  funnel  I  will  hold  the  mouth  of  this 
paper  balloon.  The  heated  air  ascending  from  the  burning 
tow  enters  the  balloon,  causes  it  to  swell ;  its  tendency  to 
rise  is  already  manifest.  I  let  it  go,  and  thus  it  sails  aloft 
till  it  strikes  the  ceiling  of  the  room. 

But  we  must  not  be  content  with  regarding  these  phe- 
nomena in  a  general  way ;  without  exact  quantitative  de- 
terminations our  discoveries  would  confound  and  bewilder 
us.  We  must  now  enquire  what  is  the  amount  of  expan- 
sion which  a  given  quantity  of  heat  is  able  to  produce  hi  a 
gas  ?  This  is  an  important  point,  and  demands  our  special 
attention.  When  we  speak-  of  the  volume  of  a  gas,  we 
should  have  no  distinct  notion  of  its  real  quantity,  if  its 
temperature  were  omitted,  the  volume  varies  so  largely 
with  the  temperature.  Take,  then,  a  measure  of  gas  at  the 
precise  temperature  of  water  when  it  begins  to  freeze,  or 
of  ice  when  it  commences  to  melt,  that  is  to  say,  at  a  tem- 
perature of  32°  Fahr.  or  0°  Cent.,  and  raise  that  volume  of 
gas  one  degree  in  temperature,  the  pressure  on  every  square 
inch  of  the  envelope  which  holds  the  gas  being  preserved 
constant.  The  volume  of  the  gas  will  become  expanded 
by  a  quantity  which  we  may  call  a ;  raise  it  another  degree 
in  temperature,  its  volume  will  be  expanded  by  2a,  a  third 
degree  will  cause  an  expansion  of  3  a,  and  so  on.  Thus,  we 
see,  that  for  every  degree  which  we  add  to  the  temperature 
of  the  gas,  it  is  expanded  by  the  same  amount.  What  is 
this  amount  ?  No  matter  what  the  quantity  of  gas  may  be 
at  the  freezing  temperature,  by  raising  it  one  degree  Fahr- 
enheit we  augment  its  volume  by  T £^th  of  its  own  amount ; 
while  by  raising  it  one  degree  Centigrade  we  augment  the 

volume  by  ^ 4  ^ rd  of  its  own  amount.     A  cubic  foot  of  gas, 
4* 


82  LECTURE  m. 

for  example,  at  0°  C.,  becomes,  on  being  heated  to  1°, 
Ig-fa  cubic  foot,  or,  expressed  in  decimals, 

1  vol.  at  0°  C.  becomes        1  -f  -00367  at  1°  C. 
at  2°  C.  it  becomes     1  -f  -00367  X  2 
at  3°  C.  it  becomes     1  -j-  -00367  X  3,  and  so  on. 

The  constant  number  -00367,  which  expresses  the  frac- 
tion of  its  own  volume,  which  a  gas,  at  the  freezing  tem- 
perature, expands  on  being  heated  one  degree,  is 
called  the  coefficient  of  expansion  of  the  gas.    Of     Fig-  21< 
course  if  we  use  the  degrees  of  Fahrenheit,  the  co- 
efficient will  be  smaller  in  the  proportion  of  9  to  5? 

This  much  made  clear,  we  shall  now  approach, 
by  slow  degrees,  an  interesting  but  difficult  sub- 
ject. Suppose  I  have  a  quantity  of  air  contained 
in  a  very  tall  cylinder,  A  B  (fig.  21),  the  transverse 
section  of  which  is  one  square  inch  in  area.  Let 
the  top  A  of  the  cylinder  be  open  to  the  air,  and 
let  P  be  a  piston,  which,  for  reasons  to  be  explained 
immediately,  I  will  suppose  to  weigh  two  pounds 
one  ounce,  and  which  moves  air-tight  and  without 
friction,  up  or  down  in  the  cylinder.  At  the  com- 
mencement of  the  experiment,  let  the  piston  be  at 
the  point  P  of  the  cylinder,  and  let  the  height  of 
the  cylinder  from  its  bottom  B  to  the  point  P  be 
273  inches,  the  air  underneath  the  piston  being  at 
a  temperature  of  0°  C.  Then,  on  heating  the  air 
from  0°  to  1°  C.  the  piston  will  rise  one  inch;  it 
will  now  stand  at  274  inches  above  the  bottom.  If 
the  temperature  be  raised  two  degrees,  the  piston 
will  stand  at  275,  if  raised  three  degrees  it  will 
stand  at  276,  if  raised  ten  degrees  it  will  stand  at 
283,  if  100  degrees  it  will  stand  at  373  inches 
above  the  bottom ;  finally,  if  the  temperature  were  raised 
to  273°  C.,  it  is  quite  manifest  273  inches  would  be  added 
'  *  See  Note  (5)  at  the  end  of  this  Lecture. 


WOKK   DONE  BY   EXPANDING   GAS.  83 

to  the  height  of  the  column,  or,  in  other  words,  by  heating 
the  air  to  273°  C.,  its  volume  would  be  doubled. 

It  is  evident  that  the  gas,  in  this  experiment,  executes 
work.  In  expanding  from  p  upwards,  it  has  to  overcome 
the  downward  pressure  of  the  atmosphere,  which  amounts 
to  15  Ibs.  on  every  square  inch,  and  also  the  weight  of  the 
piston  itself,  which  is  2  Ibs.  1  oz.  Hence,  the  section  of 
the  cylinder  being  one  square  inch  in  area,  in  expanding 
from  P  to  P'  the  work  done  by  the  gas  is  equivalent  to  the 
raising  a  weight  of  17  Ibs.  1  oz.,  or  273  ounces,  to  a  height 
of  273  inches.  It  is  just  the  same  as  what  it  would  accom- 
plish, if  the  air  above  P  were  entirely  abolished,  and  a  pis- 
ton weighing  17  Ibs.  1  oz.  were  placed  at  P. 

Let  us  now  alter  our  mode  of  experiment,  and  instead 
of  allowing  our  gas  to  expand  when  heated,  let  us  oppose 
its  expansion  by  augmenting  the  pressure  upon  it.  In 
other  words,  let  us  keep  its  volume  constant  while  it  is 
being  heated.  Suppose,  as  before,  the  initial  temperature 
of  the  gas  to  be  0°  C.,  the  pressure  upon  it,  including  the 
weight  of  the  piston  P,  being,  as  formerly,  273  ounces. 
Let  us  warm  the  gas  from  0°  C.  to  1°  C. ;  what  weight 
must  we  add  to  P  in  order  to  keep  its  volume  constant  ? 
Exactly  one  ounce.  But  we  have  supposed  the  gas,  at  the 
commencement,  to  be  under  a  pressure  of  273  ounces,  and 
the  pressure  it  sustains  is  the  measure  of  its  elastic  force ; 
hence,  by  being  heated  one  degree,  the  elastic  force  of  the 
gas  has  augmented  by  -^rd  of  what  it  possessed  at  0°. 
If  we  warm  it  2°,  2  ozs.  must  be  added  to  keep  its  volume 
constant ;  if  3°,  3  ozs.  must  be  added.  And  if  we  raise  its 
temperature  273°,  we  should  have  to  add  273  ozs. ;  that  is, 
we  should  have  to  double  the  original  pressure  to  keep  the 
volume  constant. 

It  is  simply  for  the  sake  of  clearness,  and  to  avoid  frac- 
tions in  our  reflections,  that  I  have  supposed  the  gas  to  be 
under  the  original  pressure  of  273  ozs.  No  matter  what  its 


84:  LECTUKE   HI. 

pressure  may  be,  the  addition  of  1°  C.  to  its  temperature 
produces  an  augmentation  of  ^-^rd  of  the  elastic  force 
which  the  gas  possesses  at  the  freezing  temperature ;  and 
by  raising  its  temperature  273°,  while  its  volume  is  kept 
constant,  its  elastic  force  is  doubled.  Let  us  now  compare 
this  experiment  with  the  last  one.  There  we  heated  a  cer- 
tain amount  of  gas  from  0°  to  273°,  and  doubled  its  vol- 
ume by  so  doing,  the  double  volume  being  attained  while 
the  gas  lifted  a  weight  of  273  ozs.  to  a  height  of  273 
inches.  Here  we  heat  the  same  amount  of  gas  from  0°  to 
273°,  but  we  do  not  permit  it  to  lift  any  weight.  We 
keep  its  volume  constant.  The  quantity  of  matter  heated 
in  both  cases  is  the  same ;  the  temperature  to  which  it  is 
heated  is  in  both  cases  the  same ;  but  are  the  absolute 
quantities  of  heat  imparted  in  both  cases  the  same  ?  By 
no  means.  Supposing  that  to  raise  the  temperature  of  the 
gas,  whose  volume  is  kept  constant,  273°,  10  grains  of 
combustible  matter  are  necessary ;  then  to  raise  the  tem- 
perature of  the  gas  whose  pressure  is  kept  constant  an 
equal  number  of  degrees,  would  require  the  consumption 
of  14£  grains  of  the  same  combustible  matter.  The  heat 
produced  by  the  combustion  of  the  additional  4%  grains,  in 
the  latter  case,  is  entirety  consumed  in  lifting  the  weight. 
Using  the  accurate  numbers,  the  quantity  of  heat  applied 
when  the  volume  is  constant,  is  to  the  quantity  applied 
when  the  pressure  is  constant,  in  the  proportion  of 

1  to  1-421. 

This  extremely  important  fact  constitutes  the  basis 
from  which  the  mechanical  equivalent  of  heat  was  first  cal- 
culated. And  here  wre  have  reached  a  point  which  is  wor- 
thy of,  and  which  will  demand,  your  entire  attention.  I 
will  endeavour  to  make  this  calculation  before  you. 

Let  c  (fig.  2 la)  be  a  cylindrical  vessel  with  a  base  one 
square  foot  in  area.  Let  P  P  mark  the  upper  surface  of  a 


HEAT  IMPARTED  TO  GAS  AT  A  CONSTANT  VOLUME.       85 

cubic  foot  of  air  at  a  temperature  of  32°  Fahr.  The  height 
A  P  will  be  then  one  foot.  Let  the  air  be  heated  till  this 
volume  is  doubled  ;  to  effect  this  it  must, 
as  before  explained,  be  raised  273°  C.,  or 
490°  F.  in  temperature  ;  and,  when  ex- 
panded, its  upper  surface  will  stand  at  P' 
p',  one  foot  above  its  initial  position.  But 
in  rising  from  p  p  to  p'  p'  it  has  forced 
back  the  atmosphere,  which  exerts  a  pres- 
sure of  15  Ibs.  on  every  square  inch  of  its 
upper  surface ;  in  other  words,  it  has  lifted 
a  weight  of  144  x  15  =  2,160  Ibs.  to  a 
height  of  one  foot. 
The  4  capacity '  for  heat  of  the  air  thus  expanding  is 
0*24  ;  water  being  unity.  The  weight  of  our  cubic  foot  of 
air  is  T29  oz.,  hence  the  quantity  of  heat  required  to  raise 
1'29  oz.  of  air  490°  Fahr.  would  raise  a  little  less  than  one- 
fourth  of  that  weight  of  water  490°.  The  exact  quantity 
of  water  equivalent  to  our  1-29  oz.  of  air  is  1-29  X  0'24  = 
0-31  oz. 

But  0-31  oz.  of  water,,  heated  to  490°,  is  equal  to  152 
ozs.  or  9£  Ibs.  heated  1°.  Thus  the  heat  imparted  to  our 
cubic  foot  of  air,  in  order  to  double  its  volume,  and  enable 
it  to  lift  a  weight  of  2,160  Ibs.  one  foot  high,  would  be 
competent  to  raise  9^  Ibs.  of  water  one  degree  in  tempera- 
ture. 

The  air  has  here  been  heated  under  a  constant  pressure, 
and  we  have  learned,  that  the  quantity  of  heat  necessary  to 
raise  the  temperature  of  a  gas  under  constant  pressure  a 
certain  number  of  degrees,  is  to  that  required  to  raise  the 
gas  to  the  same  temperature,  when  its  volume  is  kept  con- 
stant, in  the  proportion  of  1*42  :  1  ;  hence  we  have  the 
statement — 

Ibs.       Ibs. 

1-42 :  1  =  9-5:  6'7 


86  LECTURE  m. 

which  shows  that  the  quantity  of  heat  necessary  to  aug- 
ment the  temperature  of  our  cubic  foot  of  air,  at  constant 
volume,  490°,  would  heat  6*7  Ibs.  of  water  1°. 

Deducting  6'7  Ibs.  from  9'5  Ibs.,  we  find  that  the  excess 
of  heat  imparted  to  the  air,  in  the  case  where  it  is  permit- 
ted to  expand,  is  competent  to  raise  2'8  Ibs.  of  water  1°  in 
temperature. 

As  explained  already,  this  excess  is  employed  to  lift  the 
weight  of  2,160  Ibs.  one  foot  high.  Dividing  2,100  by  2*8, 
we  find  that  a  quantity  of  heat  sufficient  to  raise  one  pound 
of  water  1°  Fahr.  in  temperature,  is  competent  to  raise  a 
weight  of  771*4  Ibs.  a  foot  high. 

This  method  of  calculating  the  mechanical  equivalent 
of  heat  was  followed  by  Dr.  Mayer,  a  physician  in  Heil- 
bron,  Germany,  in  the  spring  of  1842. 

Mayer's  first  paper  contains  merely  an  indication  of  the 
way  in  which  he  had  found  the  equivalent ;  but  does  not 
contain  the  calculation.  The  paper  was  evidently  a  kind 
of  preliminary  note,  from  which  date  might  be  taken.  In 
it  were  enunciated  the  convertibility  and  indestructibility 
of  force,  and  its  author  referred  to  the  mechanical  equiva- 
lent of  heat,  merely  in  illustration  of  his  principles.  Had 
this  first  paper ^tood  alone,  Mayer's  relation  to  the  dynami- 
cal theory  of  heat  would  be  very  different  from  what  it 
now  is ;  but  in  1845  he  published  an  Essay  on  Organic 
Motion,  which,  though  exception  might  be  taken  to  it  here 
and  there,  is,  on  the  whole,  a  production  of  extraordinary 
merit.  This  was  followed  in  1848  by  an  Essay  on  '  Celes- 
tial Dynamics,'  in  which,  with  remarkable  boldness,  saga- 
city, and  completeness,  he  developed  the  meteoric  theory 
of  the  sun.  Taking  him  all  in  all,  the  right  of  Mayer  to 
stand,  as  a  man  of  true  genius,  in  the  front  rank  of  the 
founders  of  the  dynamical  theory  of  heat,  cannot  be  dis- 
puted. 

On  August  21,  1843,  Mr.  Joule  communicated  a  paper 


JOULE'S  EXPEKIMENTS.  87 

to  the  British  Association,  then  meeting  at  Cork,  and  in  the 
third  part  of  this  paper*  he  describes  a  series  of  expert 
ments  on  magneto-electricity,  executed  with  a  view  to  de- 
termine the  '  mechanical  value  of  heat.'  The  results  of  this 
elaborate  investigation  gave  the  following  weights  raised 
one  foot  high,  as  equivalent  to  the  warming  of  1  Ib.  of 
water  1°  Fahr. 

1.  896  Ibs.  5.  1026  Ibs. 

2.  1001  „  6.  587  „ 

3.  1040  „  7.  742  „ 

4.  910  „  8.  860  „ 

In  1844  Mr.  Joule  deduced  from  experiments  on  the 
condensation  of  air,  the  following  equivalents  to  1  Ib.  of 
water  heated  1°  Fahr.f 

823  foot  pounds 

795 

820 

814 

760 

As  the  experience  of  the  experimenter  increased,  we 
find  that  the  coincidence  of  his  results  becomes  closer.  In 
1845  Mr.  Joule  deduced  from  experiments  with  water,  agi- 
tated by  a  paddle-wheel,  an  equivalent  of 

890  foot  pounds. 

Summing  up  his  results  in  1845,  and  taking  the  mean, 
he  found  the  equivalent  to  be 

817  foot  pounds. 

In  1847  he  found  the  mean  of  two  experiments  to  give 
as  equivalent 

781*8  foot  pounds. 

*  Phil.  Mag.,  1813,  vol.  xxiii.  p.  435. 
f  See  Note  (6)  at  the  end  of  this  Lecture. 


88  LECTUKE   HI. 

Finally,  in  1849,  applying  all  the  precautions  suggested 
by  seven  years'  experience,  he  obtained  the  following  num- 
bers for  the  mechanical  equivalent  of  heat : — 

772-692,  from  friction  of  water,      mean  Of  40  experiments        * 
774-083,         „          „      mercury,         „         50         „ 
774-987,         „          „      cast-iron,         „         20        „ 

For  reasons  assigned  in  his  paper,  Mr.  Joule  fixes  the 
exact  equivalent  of  heat  at 

773  foot  pounds. 

According  to  the  method  pursued  by  Mayer,  in  1842, 
the  mechanical  equivalent  of  heat  is 

771*4  foot  pounds. 

Such  a  coincidence  relieves  the  mind  of  every  shade  of 
uncertainty,  regarding  the  correctness  of  our  present  me- 
chanical equivalent  of  heat. 

Do  I  refer  to  these  things  in  order  to  exalt  Mayer,  at 
the  expense  of  Joule  ?  It  is  far  from  my  intention  to  do 
so.  The  man  who  through  long  years,  without  encourage- 
ment, and  in  the  face  of  difficulties  which  might  well  be 
deemed  insurmountable,  could  work  with  such  unswerving 
steadfastness  of  purpose  to  so  triumphant  an  issue,  is  safe 
from  depreciation.  And  it  is  not  the  experiments  alone, 
but  the  spirit  which  they  incorporate,  and  the  applications 
which  their  author  made  of  them,  that  entitle  Mr.  Joule  to 
a  place  in  the  foremost  rank  of  physical  philosophers. 
Mayer's  labours  have,  in  some  measure,  the  stamp  of  a  pro- 
found intuition,  which  rose,  however,  to  the  energy  of  un- 
doubting  conviction  in  the  author's  mind.  Joule's  labours, 
on  the  contrary,  are  an  experimental  demonstration.  True 
to  the  speculative  instincts  of  his  country,  Mayer  drew 
large  and  weighty  conclusions  from  slender  premises,  while 
the  Englishman  aimed,  above  all  things,  at  the  firm  estab- 
lishment of  facts.  And  he  did  establish  them.  The  future 


JOULE'S   EXPEKIMENTS. 


89 


historian  of  science  will  not,  I  think,  place  chess  men  in 
antagonism.  To  each  belongs  a  reputation  which  will  not 
quickly  fade,  for  the  share  he  has  had,  not  only  in  .estab- 
lishing the  dynamical  theory  of  heat,  but  also  in  leading 
the  way  towards  a  right  appreciation  of  the  general  energies 
of  the  universe. 

Let  us  now  check  our  conclusion  regarding  the  influence 
which  the  performance  of  work  has  on  the  quantity  of  heat 
communicated  to  a  gas.  Is  it  not  possible  to  allow  a  gas 
to  expand,  without  performing  work  ?  This  question  is  an- 
swered by  the  following  important  experiment,  which  was 
first  made  by  Gay  Lussac.  I  have  here  two  copper  vessels, 
A,  B  (fig.  22),  of  the  same  size,  one  of  which,  A,  is  exhaust- 
ed, and  the  other,  B,  filled  with  air.  I  turn  the  cock  c ; 
the  air  rushes  out  of  B  into  A,  until  the  same  pressure  exists 
in  both  vessels.  Now  the  air  in 
driving  its  own  particles  out  of  B 
performs  work,  and  experiments 
which  we  have  already  made  in- 
form us,  that  the  residue  of  air 
which  remains  in  B  must  be  chill- 
ed. The  particles  of  air  enter  A 
with  a  certain  velocity,  to  generate 
which  the  heat  of  the  air  in  B  has 
been  sacrificed ;  but  they  immedi- 
ately strike  against  the  interior 
surface  of  A,  their  motion  of  trans- 
lation is  annihilated,  and  the  exact  quantity  of  heat  lost  by 
B  appears  in  A.  Mix  the  contents  of  A  and  B  together,  and 
you  have  air  of  the  original  temperature.  There  is  no  work 
performed,  and  there  is  no  heat  lost.  Mr.  Joule  made  this 
experiment  by  compressing  twenty-two  atmospheres  of  air 
into  qne  of  his  vessels,  while  the  other  was  exhausted.  On 
surrounding  both  vessels  by  water,  kept  properly  agitated, 
no  augmentation  of  temperature  was  observed  in  the  water, 


90  LECTURE  m. 

when  the  gas  was  allowed  to  stream  from  one  vessel  into 
the  other.*  In  like  manner,  supposing  the  top  of  the  cylin- 
der (fig.  2t)  to  be  closed,  and  the  half  above  the  piston  a 
perfect  vacuum ;  and  suppose  the  air  in  the  lower  half  to 
be  heated  273°,  its  volume  being  kept  constant.  If  the 
pressure  were  removed,  the  air  would  expand  and  fill  the 
cylinder ;  the  lower  portion  of  the  column  would  thereby 
be  chilled,  but  the  upper  portion  would  be  heated,  and 
mixing  both  portions  together,  we  should  have  the  whole 
column  at  a  temperature  of  273°.  In  this  case  we  raise  the 
temperature  of  the  gas  from  0°  to  273°,  and  afterwards  al- 
low it  to  double  its  volume ;  the  state  of  the  gas  at  the 
commencement,  and  at  the  end,  is  the  same  as  when  the 
gas  expands  against  a  constant  pressure,  or  lifts  a  constant 
weight ;  but  the  absolute  quantity  of  heat  in  the  latter  case 
is  1*421  times  that  employed  in  the  former,  the  difference 
being  due  to  the  fact  that  the  gas,  in  the  one  case,  per- 
forms mechanical  work,  and  in  the  other  not. 

We  are  taught  by  this  experiment  that  mere  rarefaction 
is  not  of  itself  sufficient  to  produce  a  lowering  of  the  mean 
temperature  of  a  mass  of  air.  It  was,  and  is  still,  a  current 
notion,  that  the  mere  expansion  of  a  gas  produced  refriger- 
ation, no  .matter  how  that  expansion  was  effected.  The 
coldness  of  the  higher  atmospheric,  regions  was  accounted 
for  by  reference  to  the  expansion  of  the  air.  It  was 
thought  that  what  we  have  called  the  '  capacity  for  heat ' 
was  greater  in  the  case  of  the  rarefied  than  of  the  unrare^ 
fied  gas.  But  the  refrigeration  which  accompanies  expan- 
sion is,  in  reality,  due  to  the  consumption  of  heat  in  the 
performance  of  work  by  the  expanding  gas.  Where  no 
work  is  performed  there  is  no  absolute  refrigeration. 

All  this  needs  reflection  to  arrive  at  clearness,  but  every 
effort  of  this  kind  which  you  make  will  render  your  subse- 

*  Phil.  Mag.  1845,  vol.  xxvi.  p.  378. 


EXPANSION  OF  LIQUIDS.  91 

quent  efforts  easier,  and  should  you  "fail,  at  present,  to  gain 
clearness  of  comprehension,  I  repeat  my  recommendation 
of  patience.  Do  not  quit  this  portion  of  the  subject  with- 
out an  effort  to  comprehend  it — wrestle  with  it  for  a  time, 
but  do  not  despair  if  you  fail  to  arrive  at  clearness. 

I  have  now  to  direct  your  attention  to  one  other  inter- 
esting question.  We  have  seen  the  elastic  force  of  our  gas 
augmented  by  an  increase  of  temperature.  In  an  inflexible 
envelope  we  have,  for  every  degree  of  temperature,  a  cer- 
tain definite  increment  of  elastic  force,  due  to  the  augment- 
ed energy  of  the  gaseous  projectiles.  Reckoning  from  0° 
C.  upwards,  we  find  that  every  degree  added  to  the  tem- 
perature produces  an  augmentation  of  elastic  force,  equal 
to  aT^rd  of  that  which  the  gas  possesses  at  0°,  and  hence, 
that  by  imparting  273°  we  double  the  elastic  force.  Sup- 
posing the  same  law  to  hold  good  when  we  reckon  from  0° 
doicnwards — that  for  every  degree  of  temperature  with- 
drawn from  the  gas  we  diminish  its  elastic  force,  or  the 
motion  which  produces  it,  by  2T^r<^  °^  what  it  possesses  at 
0°,  it  is  manifest  that  at  a  temperature  of  273°  Centigrade 
below  0°  we  should  cease  to  have  any  elastic  force  what- 
ever. The  motion  to  which  the  elastic  force  is  due  must 
here  vanish,  and  we  reach  what  is  called  the  absolute  zero 
of  temperature. 

No  doubt,  practically,  every  gas  deviates  from  the 
above  law  of  contraction  before  it  sinks  so  low,  and  it 
would  become  solid  before  reaching — 273°  C.,  or  the  abso- 
lute zero.  This  is  considerably  below  any  temperature 
which  we  have  as  yet  been  able  to  obtain. 

I  will  not  subject  your  minds  to  any  further  strain  in 
connection  with  this  subject  to-day,  but  will  now  pass  on 
to  illustrate  experimentally  the  expansion  of  liquids  by 
heat. 

Here  is  a  Florence  flask  filled  with  alcohol,  and  tightly 
corked;  through  the  cork  a  tube,  t'  (fig.  23),  passes. water* 


92 


LECTURE  IH. 


tight,  and  the  liquid  rises  a  foot  or  so  in  this  tube.  I  will 
heat  this  flask,  the  alcohol  will  expand,  and  it  will  rise  in 
the  tube.  But  I  wish  you  to  see  it  rising,  and  to  enable 
you  to  do  so  I  will  place  the  tube  1 1'  in  front  of  the  elec- 
tric lamp  E,  and  send  a  strong  beam  of  light  across  it,  at 

Fig.  23. 


the  place  £',  where  the  liquid  column  ends ;  I  thus  illumi- 
nate the  tube  and  column.  In  front  of  the  tu]|p  I  place  this 
lens  L,  and  arrange  its  distance  so  that  it  shall  cast  an  en- 
larged image  i  i,  of  the  column  upon  the  screen.  You  now 
see  clearly  where  the  column  ends  ;  you  see  this  quivering 
of  the  top  of  the  column,  and  if  it  moves,  you  will  be  able 
to  see.  its  motion.  I  now  fill  this  beaker,  B,  with  hot  wa- 


EXPANSION  OF  LIQUIDS.  93 

ter,  and  I  will  raise  the  beaker  so  that  the  hot  water  shall 
surround  the  Florence  flask.  It  is  needless  to  say  that  the 
image  upon  the  screen  is  inverted,  and  that  when  the  liquid 
expands,  the  top  of  the  column  will  descend  along  the 
screen.  Observe  the  experiment  from  the  commencement ; 
the  flask  is  now  in  the  hot  water,  and  the  head  of  our  col- 
umn ascends,  as  if  the  liquid  contracted.  Now  it  stops 
and  commences  to  descend,  and  it  will  continue  to  do  so 
permanently.  But  why  the  first  ascent  ?  It  is  not  due  to 
the  contraction  of  the  liquid,  but  to  the  momentary  expan- 
sion of  the  flask,  to  which  the  heat  is  first  communicated. 
The  glass  expands  before  the  heat  can  fairly  reach  the 
liquid,  and  hence  the  column  falls  ;  but  soon  the  expansion 
of  the  liquid  exceeds  that  of  the  glass,  and  the  column 
rises.  Two  things  are  here  illustrated ;  the  expansion  of 
the  solid  glass  by  heat,  and  the  fact  that  the  observed  dila- 
tation of  ;the  liquid  "does  not  give  us  its  true  augmentation 
of  volume,  but  only  the  difference  of  dilatation  between 
the  glass  and  it. 

I  have  here  another  flask  filled  with  water,  exactly 
similar  in  size  to  the  former,  and  furnished  with  a  similar 
tube.  I  place  it  in  the  same  position,  and  repeat  with  it 
the  experiment  made  with  the  alcohol.  You  see,  first  of  all, 
the  transitory  effect  due  to  the  expansion  of  the  glass,  and 
afterwards,  the  permanent  expansion  of  the  liquid  ;  but  you 
can  observe  that  the  latter  proceeds  much  more  slowly  than 
in  the  case  of  alcohol ;  the  alcohol  expands  more  speedily 
than  the  water.  Now  we  might  go  over  a  hundred  liquids 
in  this  way,  and  find  them  all  expanding  by  heat,  and  we 
might  thus  be  led  to  conclude  that  expansion  by  heat  is  a 
law  without  exception ;  but  we  should  err  in  this  conclu- 
sion. And  it  is  really  to  illustrate  an  exception  of  this  kind 
that  I  have  introduced  this  flask  of  water.  I  will  cool  the 
flask  by  plunging  it  into  a  substance  somewhat  colder  than 
water,  when  it  first  freezes.  This  substance  I  obtain  by 


94  LECTURE  HI. 

mixing  pounded  ice  with  salt.  You  see  the  column  gradu- 
ally sinking,  the  heat  is  being  given  up  to  the  freezing  mix- 
ture, and  the  water  contracts.  This  contraction  is  now  very 
slow,  and  now  it  stops  altogether.  A  slight  motion  com- 
mences in  the  opposite  direction,  and  now  the  liquid  is  vis- 
ibly expanding.  I  stir  the  freezing  mixture,  so  as  to  bring 
colder  portions  of  it  into  contact  with  the  flask ;  the  colder 
the  mixture  the  quicker  is  the  expansion.  Here  then  we 
have  Nature  stopping  in  her  ordinary  course,  and  reversing 
her  ordinary  habits.  The  fact  is,  that  the  water  goes  on 
contracting  till  it  reaches  a  temperature  of  39°  Fahr.,  or  4° 
Cent.,  at  which  point  the  contraction  ceases.  This  is  the 
so-called  point  of  maximum  density  of  the  water ;  from 
this  downwards,  to  its  freezing  point,  the  liquid  expands  ; 
and  when  it  is  converted  into  ice,  the  expansion  is  large 
and  sudden.  Ice,  we  know,  swims  upon  water,  being 
lightened  by  this  expansion.  If  I  now  apply  heat,  the 
series  of  changes  are  reversed :  the  column  descends,  show- 
ing the  contraction  of  the  liquid  by  heat.  After  a  time  the 
contraction  ceases,  and  permanent  expansion  sets  in. 

The  force  with  which  these  molecular  changes  are 
effected  is  all  but  irresistible.  The  changes  usually  occur 
under  conditions  which  allow  us  no  opportunity  of  observing 
the  energy  involved  in  their  accomplishment.  But  to  give 
you  an  example  of  this  energy,  I  have  confined  a  quantity 
of  water  in  this  iron  bottle.  The  iron  is  fully  half  an  inch 
thick,  and  the  quantity  of  water  is  small,  though  sufficient 
to  fill  the  bottle.  The  bottle  is  closed  by  a  screw  firmly 
fixed  in  its  neck.  I  have  here  a  second  bottle  of  the  same 
kind,  and  prepared  in  a  similar  manner.  Both  of  them  I 
place  in  this  copper  vessel,  and  surround  them  with  a  freez- 
ing mixture.  They  cool  gradually,  the  water  within  ap- 
proaches its  point  of  maximum  density ;  no  doubt,  at  this 
moment,  the  water  does  not  quite  fill  the  bottle,  a  small 
vacuous  space  exists  within.  But  soon  the  contraction 


DEPOETMENT   OF  WATEE.  95 

ceases,  and  expansion  sets  in ;  the  vacuous  space  is  slowly 
filled,  the  water  gradually  changes  from  liquid  to  solid ;  in 
doing  so  it  requires  more  room,  which  the  rigid  iron  re- 
fuses to  grant.  But  its  rigidity  is  powerless  in  the  pres- 
ence of  the  atomic  forces.  These  atoms  are  giants  in  dis- 
guise ;  you  hear  that  sound ;  the  bottle  is  shivered  by  the 
crystallising  molecules — there  goes  the  other  ;  and  here  are 
the  fragments  of  the  vessels,  which  show  their  thickness, 
and  impress  you  with  the  might  of  that  energy  by  which 
they  were  thus  riven.* 

You  have  now  no  difficulty  in  understanding  the  effect 
of  frosty  weather  upon  the  water  pipes  of  your  houses.  I 
have  here  a  number  of  pieces  of  such  pipes,  all  rent.  You 
become  first  sensible  of  the  damage  when  the  thaw  sets  in, 
but  the  mischief  is  really  done  at  the  time  of  freezing  ;  the 
pipes  are  then  rent,  and  through  the  rents  the  water  es- 
capes, when  the  solid  within  is  liquefied. 

It  is  hardly  necessary  for  me  to  say  a  word  on  the  im- 
portance of  this  property  of  water  in  the  economy  of  na- 
ture. Suppose  a  lake  exposed  to  a  clear  wintry  sky ;  the 
superficial  water  is  chilled,  contracts,  becomes  thus  heavier, 
and  sinks  by  its  superior  weight,  its  place  being  supplied 
by  the  lighter  water  from  below.  In  time  this  is  chilled, 
and  sinks  in  turn.  Thus  a  circulation  is  established,  the 
cold,  dense  water  descending,  and  the  lighter  and  warmer 
water  rising  to  the  top.  Supposing  this  to  continue,  even 
after  the  first  pellicles  of  ice  were  formed  at  the  surface ; 
the  ice  would  sink  as  it  was  formed,!  and  the  process 

*  Metal  cylinders,  an  inch  in  thickness,  are  unable  to  resist  the  decom- 
posing force  of  a  small  galvanic  battery.  M.  Gassoit  has  burst  many  such 
cylinders  by  electrolytic  gas. 

f  Prof.  William  Thomson  has  recently  raised  a  point  which  deserves 
the  grave  consideration  of  theoretic  geologists :  Supposing  the  constituents 
of  the  earth's  crust  to  contract  on  solidifying,  as  the  experiments  thus  far 
made  indicate,  a  breaking  in,  and  sinking  of  the  crust  would  assuredly 


96  LECTURE  m. 

would  not  cease  until  the  entire  water  of  the  lake  would  be 
solidified.  Death  to  every  living  thing  in  the  water  would 
be  the  consequence.  But  just  when  matters  become  critical, 
Nature  steps  aside  from  her  ordinary  proceeding,  causes 
the  water  to  expand  by  cooling,  and  the  cold  water  swims 
like  a  scum  on  the  surface  of  the  warmer  water  under- 
neath. Solidification  ensues,  but  the  solid  is  much  lighter 
than  the  subjacent  liquid,  and  the  ice  forms  a  protecting 
roof  over  the  living  things  below. 

Such  facts  naturally  and  rightly  excite  the  emotions ; 
indeed,  the  relations  of  life  to  the  conditions  of  life — the 
general  adaptation  of  means  to  ends  in  Nature,  excite,  in 
the  profoundest  degree,  the  interest  of  the  philosopher. 
But  in  dealing  with  natural  phenomena,  the  feelings  must 
be  carefully  watched.  They  often  lead  us  unconsciously 
to  overstep  the  bounds  of  fact.  Thus,  I  have  heard  this 
wonderful  property  of  water  referred  to  as  an  irresistible 
proof  of  design,  unique  of  its  kind,  and  suggestive  of  pure 
benevolence.  4  Why,'  it  is  urged,  '  should  this  case  of  wa- 
ter stand  out  isolated,  if  not  for  the  purpose  of  protecting 
Nature  against  herself  ?  '  The  fact,  however,  is,  that  the 
case  is  not  an  isolated  one.  You  see  this  iron  bottle,  rent 
from  neck  to  bottom ;  I  break  it  with  this  hammer,  and 
you  see  a  core  of  metal  within.  This  is  the  metal  bismuth, 
which,  when  it  was  in  a  molten  condition,  I  poured  into 
this  bottle,  and  closed  the  bottle  by  a  screw,  exactly  as  in 
the  case  of  the  water.  The  metal  cooled,  solidified,  ex- 
panded, and  the  force  of  its  expansion  was  sufficient  to 
burst  the  bottle.  There  are  no  fish  here  to  be  saved,  still 
the  molten  bismuth  acts  exactly  as  the  water  acts.  Once 
for  all,  I  would  say  that  the  natural  philosopher,  as  such, 

follow  its  formation.  Under  these  circumstances,  it  is  extremely  difficult 
to  conceive  that  a  solid  shell  should  be  formed,  as  is  generally  assumed, 
round  a  liquid  nucleus. 


EXPANSION   OF   SOLIDS.  97 

has  nothing  to  do  with  purposes  and  designs.  His  voca- 
tion is  to  enquire  what  Nature  is,  not  why  she  is  ;  though 
he,  like  others,  and  he,  more  than  others,  must  stand  at 
times  rapt  in  wonder  at  the  mystery  in  which  he  dwells, 
and  towards  the  final  solution  of  which  his  studies  furnish 
him  with  no  clue. 

We  must  now  pass  on  to  the  expansion  of  solid  bodies, 
by  heat,  and  I  will  illustrate  it  in  this  way :  I  have  here 
two  wooden  stands,  A  and  B  (fig.  24),  with  plates  of  brass, 
p  p\  riveted  against  them.  I  hold  in  my  hand  two  bars 
of  equal  length,  one  of  brass,  the  other  of  iron,  and  these, 
as  you  observe,  are  not  sufficiently  long  to  stretch  from 

Fig.  24. 


stand  to  stand.  I  will  support  them  on  two  little  projec- 
tions of  wood  attached  to  the  stand  atp  andy.  I  connect 
one  of  the  plates  of  brass,  p,  with  one  pole  of  a  small  vol- 
taic battery,  D,  and  from  the  other,  p',  a  wire  proceeds  to 


98  LECTUEE  ra. 

the  little  instrument  c,  which  you  see  in  front  of  the  table  ; 
and  again  from  that  instrument  a  wire  returns  direct  to  the 
other  pole  of  the  battery.  The  instrument  in  front  con- 
sists merely  of  an  arrangement  to  support  a  spiral  c  of  pla- 
tinum wire,  which  will  glow  with  a  pure  white  light  when 
the  current  from  D  passes  through  it.  At  the  present  mo- 
ment the  only  break  in  the  circuit  is  due  to  the  insufficient 
length  of  the  bars  of  brass  and  iron  to  bridge  the  space  from 
stand  to  stand.  Underneath  the  bars  is  a  row  of  gas  jets, 
which  I  will  now  ignite ;  the  bars  are  heated,  the  metals 
expand,  and  I  expect  that  in  a  few  minutes  they  will 
stretch  quite  across  from  plate  to  plate ;  when  this  occurs, 
the  current  will  pass,  and  the  fact  of  the  gap  being  bridged 
will  be  declared  by  the  sudden  glowing  of  the  platinum 
spiral.  It  is  still  non-luminous,  the  bridge  is  not  yet  com- 
plete ;  but  now  it  brightens  up,  showing  that  one,  or  both, 
of  these  bars  have  expanded  so  as  to  stretch  quite  across 
from  stand  to  stand.  Which  of  the  bars  is  it  ?  I  remove 
the  iron,  but  the  platinum  still  glows :  I  restore  the  iron, 
and  remove  the  brass ;  the  light  disappears.  It  was  the 
brass  that  bridged  the  gap.  So  that  we  have  here  an  illus- 
tration, not  only  of  the  general  fact  of  expansion,  but  also 
of  the  fact  that  different  bodies  expand  in  different  degrees. 
The  expansion  of  both  brass  and  iron  is  very  small :  and 
various  instruments  have  been  devised  to  measure  the  ex- 
pansion. Such  instruments  go  under  the  general  name  of 
pyrometers.  But  I  have  here  a  means  of  multiplying  the 
effect,  far  more  powerful  than  the  ordinary  pyrometer. 
Here  is  a  solid  upright  bar  of  iron  two  feet  long,  and  on  a 
mirror  connected  with  the  top  of  the  bar  I  throw  a  beam 
of  light  from  the  electric  lamp,  which  beam  is  reflected  to 
the  upper  part  of  the  wall.  If  the  bar  shorten,  the  mirror 
will  turn  in  one  direction :  if  it  lengthen,  the  mirror  will 
turn  in  the  opposite  direction.  Every  movement  of  the 
mirror,  however  slight,  is  multiplied  by  this  long  index  of 


ENERGY   OF  FORCE  OF   EXPANSION.  99 

light ;  which,  besides  its  length,  has  the  advantage  of  mov- 
ing with  twice  the  angular  velocity  of  the  mirror.  Even 
the  breath,  projected  against  this  massive  bar  of  iron,  pro- 
duces a  sensible  motion  of  the  beam ;  and  if  I  warm  it  for 
a  moment  with  the  flame  of  a  spirit-lamp,  the  luminous  in- 
dex will  travel  downwards,  the  patch  of  light  upon  the 
wall  moving  through  a  space  of  full  thirty  feet.  I  with- 
draw the  lamp,  and  allow  the  bar  to  cool ;  it  contracts,  and 
the  patch  of  light  reascends  the  wall :  I  hasten  the  con- 
traction by  throwing  a  little  alcohol  on  the  bar  of  iron,  the 
light  moves  more  speedily  upwards,  and  now  it  occupies  a 
place  near  the  ceiling,  as  at  the  commencement  of  the  ex- 
periment.* 

I  have  stated  that  different  bodies  possess  different 
powers  of  expansion  ;f  that  brass,  for  example,  expands 
more,  on  being  heated,  than  iron.  Here  are  two  rulers, 
one  of  brass  and  the  other  of  iron,  riveted  together  so  as 
to  form,  at  this  temperature,  a  straight  compound  ruler. 
But  if  the  temperature  be  changed,  the  ruler  is  no  longer 
straight.  I  heat  it,  it  bends  in  one  direction  :  I  cool  it,  it 
bends  in  the  opposite  direction.  When  heated,  the  brass 
expands  most,  and  forms  the  convex  side  of  the  curved 
ruler.  When  cooled,  the  brass  contracts  most,  and  forms 
the  concave  side  of  the  ruler.  Facts  like  these  must,  of 
course,  be  taken  into  account,  in  structures  where  it  is  ne- 
cessary to  avoid  distortion.  The  force  with  which  bodies 
expand  when  heated,  is  quite  irresistible  by  any  mechanical 
appliances  that  we  can  make  use  of.  All  these  molecular 
forces,  though  operating  in  such  minute  spaces,  are  almost 
infinite  in  energy.  The  contractile  force  of  cooling  has 

*  The  piece  of  apparatus  with  which  this  experiment  was  made  is  in- 
tended for  a  totally  different  purpose.  I  therefore  indicate  its  principle 
merely. 

f  The  coefficients  of  expansion  of  a  few  well-known  substances  are 
given  in  the  Appendix  to  this  Lecture. 


100  LECTURE   HI. 

been  applied  by  engineers  to  draw  leaning  walls  into  an 
upright  position.  If  a  body  be  brittle,  the  heating  of  one 
portion  of  it,  producing  expansion,  may  so  press  or  strain 
another  portion,  as  to  produce  fracture.  Hot  water  poured 
into  a  glass  often  cracks  it,  through  the  sudden  expansion 
of  the  interior.  It  may  also  be  cracked  by  the  contraction 
produced  by  intense  cold. 

I  have  here  some  flasks  of  very  thick  glass,  which,  when 
blown,  were  allowed  to  cool  quickly.  The  external  por- 
tions become  first  chilled  and  rigid.  The  internal  portions 
cooled  more  gradually,  but  they  found  themselves,  on 
cooling,  surrounded,  as  it  were,  by  a  rigid  shell,  on  which 
they  exerted  the  powerful  strain  of  their  contraction.  The 
consequence  is,  that  the  superficial  portions  of  these  flasks 
are  in  such  a  state  of  tension  that  the  slightest  scratch  pro- 
duces rupture.  I  throw  into  this  glass  a  grain  of  quartz ; 
the  mere  dropping  of  the  little  bit  of  hard  quartz  into  the 
flask  causes  the  bottom  to  fly  out  of  it.  Here,  also,  I  have 
these  so-called  Rupert  drops,  or  Butch  tears,  produced 
by  glass  being  fused  to  drops,  which  are  suddenly  cooled. 
The  external  rigid  shell  has  to  bear  the  strain  of  the  inner 
contraction  ;  but  the  strain  is  distributed  so  equally  all  over 
the  surface,  that  no  part  gives  way.  But  by  simply  break- 
ing this  filament  of  glass,  which  forms  the  tail  of  the  drop, 
the  solid  mass  is  instantly  reduced  to  powder.  I  dip  the 
drop  into  a  small  flask  filled  with  water,  and  break  the  tail 
of  the  drop  outside  the  flask ;  the  drop  is  shivered  with 
such  force  that  the  shock,  transferred  through  the  water,  is 
sufficient  to  break  the  bottle  in  pieces. 

A  very  curious  effect  of  expansion  was  observed,  and 
explained,  some  years  ago  by  the  Reverend  Canon  Mosely. 
The  choir  of  Bristol  Cathedral  was  covered  with  sheet  lead, 
the  length  of  the  covering  being  60  feet,  and  its  depth  19 
feet  4  inches.  It  had  been  laid  on  in  the  year  1851,  and 
two  years  afterwards — viz.,  in  1853 — it  had  moved  bodily 


ENERGY  OF  FORCE  OF  EXPANSION.        101 

down  for  a  distance  of  eighteen  inches.  The  descent  had 
been  continually  going  on  from  the  time  the  lead  had  been 
laid  down,  and  an  attempt  made  to  stop  it  by  driving  nails 
into  the  rafters  had  failed ;  for  the.  force  with  which  the 
lead  descended  was  sufficient  to  draw  out  the  nails.  The 
roof  was  not  a  steep  one,  and  the  lead  would  have  rested 
on  it  for  ever,  without  sliding  down  by  gravity.  What, 
then,  was  the  cause  of  the  descent?  Simply  this.  The 
lead  was  exposed  to  the  varying  temperatures  of  day  and 
night.  During  the  day  the  heat  imparted  to  it  caused  it  to 
expand.  Had  it  lain  upon  a  horizontal  surface,  it  would 
have  expanded  equally  all  round,  but  as  it  lay  upon  an  in- 
clined surface,  it  expanded  more  freely  downwards  than 
upwards.  When,  on  the  contrary,  the  lead  contracted  at 
night,  its  upper  edge  was  drawn  more  easily  downwards 
than  its  lower  edge  upwards.  Its  motion  was  therefore 
exactly  that  of  a  common  earthworm ;  it  pushed  its  lower 
edge  forward  during  the  day,  and  drew  its  upper  edge 
after  it  during  the  night,  and  thus  by  degrees  it  crawled 
through  a  space  of  eighte'en  inches  in  two  years.  Every 
local  change  of  temperature  during  the  day  and  during  the 
night  contributed  also  to  the  result ;  indeed  Canon  Mosely 
afterwards  found  the  main  effect  to  be  due  to  these  quicker 
alternations  of  temperature. 

Not  only  do  different  bodies  expand  differently  by  heat, 
but  the  same  body  may  expand  differently  in  different  di- 
rections. In  crystals  the  atoms  are  laid  together  according 
to  law,  and  along  some  lines  they  are  more  closely  packed 
than  along  others.  It  is  also  likely  that  the  atoms  of  many 
crystalline  bodies  oscillate  more  freely  and  widely  in  some 
directions  than  in  others.  The  consequence  of  this  would 
be  an  unequal  expansion  by  heat  in  different  directions. 
This  crystal  1  hold  in  my  hand  (Iceland  spar)  has  been 
proved  by  Professor  Mitscherlich  to  expand  more  along  its 
crystallographic  axis  than  in  any  other  direction.  Nay, 


102  LECTUliE   HI. 

while  the  crystal  expands  as  a  whole — that  is  to  say,  while 
its  volume  is  augmented  by  heat — it  actually  contracts  in  a 
direction  at  right  angles  to  the  crystallographic  axis.  Many 
other  crystals  also  expand  differently  in  different  direction? ; 
and,  I  doubt  not,  most  organic  structures  would,  if  exam- 
ined, exhibit  the  same  fact. 

Nature  is  full  of  anomalies  which  no  foresight  can  pre- 
dict, and  which  experiment  alone  can  reveal.  From  the 
deportment  of  a  vast  number  of  bodies,  we  should  be  led 
to  conclude  that  heat  always  produces  expansion,  and  that 
cold  always  produces  contraction.  But  water  steps  in,  and 
bismuth  steps  in  to  qualify  this  conclusion.  If  a  metal  be 
compressed,  heat  is  developed  :  but  if  a  metal  wire  be 
stretched,  cold  is  developed.  Mr.  Joule  and  others  have 
worked  at  this  subject,  and  found  the  above  fact  all  but 
general. 

One  striking  exception  to  this  rule  (I  have  no  doubt 
there  are  many  others)  has  been  known  for  a  great  number 
of  years  ;  and  I  will  now  illustrate  this  exception  by  an  ex- 
periment. My  assistant  will  hand  me  a  sheet  of  India- 
rubber,  which  I  hare  placed  in  the  next  room  to  keep  it 
quite  cold.  From  this  sheet  I  cut  a  strip  three  inches  long, 
and  an  inch  and  a  half  wide ;  I  turn  my  thermo-electric  pile 
upon  its  back,  and  upon  its  exposed  face  I  lay  this  piece  of 
India-rubber.  From  the  deflection  of  the  needle,  you  see 
that  that  piece  of  rubber  is  cold.  I  now, lay  hold  of  the 
ends  of  the  strip,  suddenly  stretch  it,  and  press  it,  while 
stretched,  on  the  face  of  the  pile.  See  the  effect !  The 
needle  moves  with  energy,  and  showing  that  the  stretched 
rubber  has  heated  the  pile. 

But  one  deviation  from  a  rule  always  carries  other  de- 
viations in  its  train.  In  the  physical  world,  as  in  the  moral, 
acts  are  never  isolated.  Thus  with  regard  to  our  India- 
rubber  ;  its  deviation  from  the  rule  referred  to  is  only  part 
of  a  series  of  deviations.  In  many  of  his  investigations 


DEPORTMENT   OF   INDIA   RUBBER. 


103 


rig.  25, 


Mr.  Joule  has  been  associated 
with  an  eminent  natural  philos- 
opher— Professor  William  Thom- 
son— and  when  Mr.  Thomson  was 
made  aware  of  the  deviation  of 
India-rubber  from  an  almost  gen- 
eral rule,  he  suggested  that  the 
stretched  India-rubber  might 
shorten,  on  being  heated.  The 
test  wras  applied  by  Mr.  Joule, 
and  the  shortening  was  found  to 
take  place.  This  singular  exper- 
iment, thrown  into  a  suitable 
form,  I  will  now  perform  before 
you. 

I  fasten  to  this  arm,  a  a  (fig. 
25),  a  length  of  common  vulcan- 
ised India-rubber  tubing,  and 
stretch  it  by  a  weight,  w,  of  ten 
pounds,  to  about  three  times  its 
former  length.  Here  is  an  index, 
i  i,  formed  first  of  a  piece  of 
light  wood  moving  freely  on  a 


104  LECTURE   III. 

pivot,  and  prolonged  by  a  stout  straight  straw.  At  the  end 
of  the  straw  I  place  a  spear-shaped  piece  of  paper,  which 
can  range  over  the  graduated  circle  drawn  on  this  black 
board.  The  index  is  now  pressed  down  by  a  projection 
which  I  have  attached  to  the  weight ;  but  if  the  weight 
should  be  lifted  by  the  contraction  of  the  India-rubber,  the 
lever  will  follow  it,  being  drawn  after  it  by  a  spring,  s  s, 
which  acts  upon  the  short  arm  of  the  index.  The  India- 
rubber  tube,  you  observe,  passes  through  a  sheet  iron 
chimney,  c,  through  which  I  will  now  allow  a  current  of 
hot  air  to  ascend  from  this  lamp  L.  You  see  the  effect ; 
the  index  rises,  showing  that  the  rubber  contracts,  and  by 
continuing  to  apply  the  heat  for  a  minute  or  so,  I  cause  the 
end  of  my  index  to  describe  an  arc  fully  three  feet  long. 
I  withdraw  the  lamp,  and  as  the  India-rubber  returns  to  its 
former  temperature,  it  lengthens  ;  the  index  moves  down- 
wards, and  now  it  rests  even  below  the  position  which  it 
first  occupied. 

NOTES. 

(4)  It  is  not  difficult  to  determine  the  average  velocities  with  which 
the  particles  of  various  gases  move,  according  to  the  hypothesis  of  trans- 
lation. Taking,  for  example,  a  gas  at  the  pressure  of  an  atmosphere,  or 
of  15  Ibs.  per  square  inch,  and  placing  it  in  a  vessel  a  cubic  inch  in  size 
and  shape ;  from  the  weight  of  the  gas  we  can  calculate  the  velocity  with 
which  its  particles  must  strike  each  side  of  the  vessel  in  order  to  counter- 
act a  pressure  of  15  Ibs.  It  is  manifest  at  the  outset,  that  the  lighter  the 
gas  is,  the  greater  must  be  its  velocity  to  produce  the  required  effect. 
Accorbing  to  Clausius  (Phil.  Mag.,  1857,  vol.  xiv.  p.  124),  the  following 
are  the  average  velocities  of  the  atoms  of  oxygen  nitrogen,  and  hydrogen, 
at  the  temperature  of  melting  ice : — 

Oxygon.  .  .  .  .     •     .  .    1,51 4  feet  per  second. 

Nitrogen  .  .  .  .  .  .    1,616    "  " 

Hydrogen 6,050    "  " 

In  1848,  Mr.  Joule  found  the  velocity  of  hydrogen  atoms  to  be  6,055  feet 
per  second. 


NOTES.  105 

(5)  It  is  a  very  remarkable  and  significant  fact  that  all  permanent 
gases  should  expand  by  almost  precisely  the  same  amount  for  every  degree 
added  to  their  temperature.     We  can  deduce  from  this  with  extreme  prob- 
ability the  important  conclusion,  that  where  heat  causes  a  gas  to  expand, 
the  work  it  performs  consists  solely  in  overcoming  the  constant  pressure 
from  without — that,  in  other  words,  the  heat  is  not  interfered  with  by  the 
mutual  attraction  of  the  gaseous  molecules.    For  if  this  were  the  case,  we 
should  have  every  reason  to  expect,  in  the  case  of  different  gases,  the 
same  irregularities  of  expansion  which  we  observe  in  liquids  and  solids.    I 
said  intentionally  '  by  almost  precisely  the  same  amount,'  for  many  gases 
which  are  permanent  at  all  ordinary  temperatures  deviate  slightly  from  the 
rule.     This  will  be  seen  from  the  following  table : — 

Name  of  Gas.  Coefficient  of  Expansion. 

Hydrogen 0-00366 

Air 0-00367 

Carbonic  oxide  .......  O'OOSGT 

Carbonic  acid 0-00371 

Protoxide  of  nitrogen  ......  0-00372 

Sulphurous  acid  .  .  .  .  .  .  .  G'00390 

Here  hydrogen,  air,  and  carbonic  oxide  agree  very  closely ;  still  there 
is  a  slight  difference,  the  coefficient  for  hydrogen  being  the  least.  "We  re- 
mark in  the  other  cases  a  greater  deviation  from  the  rule ;  and  it  is  par- 
ticularly noticeable  that  the  gases  which  deviate  most  are  those  which  are 
nearest  their  point  of  liquefaction.  The  first  three  gases  in  the  table 
never  have  been  liquefied,  all  the  others  have.  They  are,  in  fact,  imper- 
fect gases,  occupying  a  kind  of  intermediate  place  between  the  liquid  and 
the  perfect  gaseous  state. 

(6)  From  the  passage  of  water  through  narrow  tubes,  Mr.  Joule  de- 
duced an  equivalent  of 

770  foot  pounds. 


APPENDIX    TO    LECTURE    III. 


FUETHEE  EEMAEKS  ON  DILATATION. 

IT  is  not  within  the  scope  of  these  lectures  to  dwell  in  detail 
on  all  the  phenomena  of  expansion  by  heat ;  but  for  the  sake  of 
my  young  readers,  I  will  supplement  this  lecture  by  a  few  addi- 
tional remarks. 

The  linear,  superficial,  or  cubic  coefficient  of  expansion,  is  that 
fraction  of  a  body's  length,  surface,  or  volume,  which  it  expands 
on  being  heated  one  degree. 

Supposing  one  of  the  sides  of  a  square  plate  of  metal,  whose 
length  is  1,  to  expand,  on  being  heated  one  degree,  by  the  quanti- 
ty a ;  then  the  side  of  the  new  square  is  1  +  a,  and  its  area  is 

1  +  2a  +  a?. 

In  the  case  of  expansion  by  heat,  the  quantity  a  is  so  small,  that 
its  square  is  almost  insensible ;  the  square  of  a  small  fraction  is, 
of  course,  greatly  less  than  the  fraction  itself.  Hence  without 
sensible  error,  we  may  throw  away  the  a?  in  the  above  expres- 
sion, and  then  we  should  have  the  area  of  the  new  square 

1  +  2a. 

2a,  then,  is  the  superficial  coefficient  of  expansion ;  hence  we  infer 
that  by  multiplying  the  linear  coefficient  by  2,  we  obtain  the  su- 
perficial coefficient. 

Suppose,  instead  of  a  square,  that  we  had  a  cube,  having  a 
side  =  1 ;  and  that  on  heating  the  cube  one  degree,  the  side  ex- 
panded to  1  +  a  ;  then  the  volume  of  the  expanded  cube  would 
be 


.  0-000017 

0-000051 

0-000051 

.  0-000029 

0-000087 

0-000089 

.  0-000023 

0-000069 

0-000069 

.  0-0000123 

0-000037 

0-000037 

.  0-0000294 

0-000088 

0-000089 

.  0-000080 

0-000024 

0-000024 

EXPANSION  :   THE  THEEMOMETEE.  107 

In  this,  as  in  the  former  case,  the  square  of  a,  and  much  more  the 
cube  of  a,  may  be  neglected,  on  account  of  their  exceeding  small- 
ness  ;  we  then  have  the  volume  of  the  expanded  cube 


that  is  to  say,  the  cubic  coefficient  of  expansion  is  found  by  treb- 
ling the  linear  coefficient. 

The  following  table  contains  the  coefficients  of  expansion,  for 
a  number  of  well-known  substances  :  — 

Copper 
Lead  . 
Tin  . 
Iron  . 
Zinc  . 
Glass  . 

The  second  column  here  gives  the  linear  coefficient  of  expansion 
for  1°  C.  ;  the  third  column  contains  this  coefficient  trebled, 
which  is  the  cubic  expansion  of  the  substance  ;  and  the  fourth 
column  gives  the  cubic  expansion  of  the  same  substance,  deter- 
mined directly  by  Professor  Kopp.*  It  will  be  seen  that  Kopp's 
coefficients  agree  almost  exactly  with  those  obtained  by  the  treb- 
ling of  the  linear  coefficients. 

The  linear  coefficient  of  glass  for  1°  C.  is 

0-0000080. 
That  of  platinum  is 

0-0000088. 

Hence  glass  and  platinum  expand  nearly  alike.  This  is  of  the 
greatest  importance  to  chemists,  who  often  find  it  necessary  to 
fuse  platinum  wires  into  their  glass  tubes.  Were  the  coefficients 
difierent,  the  fracture  of  the  glass  would  be  inevitable  during  the 
contraction. 

The  Thermometer. 

"Water  owes  its  liquidity  to  the  motion  of  heat;  when  this 
motion  sinks  sufficiently,  crystallisation,  as  we  have  seen,  sets  in. 

*  Phil.  Mag.,  1852,  vol.  iii.  p.  268. 


108  APPENDIX   TO   LECTUEE   HI. 

The  temperature  of  crystallisation  is  perfectly  constant  if  the 
water  be  kept  under  the  same  pressure.  For  example,  water  crys- 
tallises in  all  climates  at  the  sea-level,  at  a  temperature  of  32°  F., 
or  of  0°  C.  The  temperature  of  condensation  from  the  state  of 
steam  is  equally  constant,  as  long  as  the  pressure  remains  the 
same.  The  melting  of  ice  and  the  freezing  of  water  touch  each 
other,  if  I  may  use  the  expression,  at  32°  F. ;  the  condensation  of 
steam  and  the  boiling  of  water  touch  each  other  at  212° :  32°  then 
is  the  freezing  point  of  water,  and  it  is  the  melting  point  of  ice  ; 
212°  is  the  condensing  point  of  steam  and  the  boiling  point  of 
water.  Both  are  invariable  as  long  as  the  pressure  remains  the 
same.  Here,  then,  we  have  two  invaluable  standard  points  of 
temperature,  and  they  have  been  used  for  this  throughout  the 
world.  The  mercurial  thermometer  consists  of  a  bulb  and  a  stem 
with  capillary  bore.  The  bore  ought  to  be  of  aquable  diameter 
throughout.  The  bulb  and  a  portion  of  the  stem  are  filled  with 
mercury.  Both  are  then  plunged  into  melting  ice,  the  mercury 
shrinking,  the  column  descends,  and  finally  comes  to  rest.  Let 
the  point  at  which  it  becomes  stationary  be  marked;  it  is  the 
freezing  point  of  the  thermometer.  Let  the  instrument  be  now 
removed  and  thrust  into  boiling  water ;  the  mercury  expands,  the 
column  rises,  and  finally  attains  a  stationary  height.  Let  this 
point  be  marked ;  it  is  the  lotting  point  of  the  thermometer.  The 
space  between  the  freezing  point  and  the  boiling  point  has  been 
divided  by  Keaumur  into  80  equal  parts,  by  Fahrenheit  into  180 
equal  parts,  and  by  Celsius  into  100  equal  parts,  called  degrees.  The 
thermometer  of  Celsius  is  also  called  the  Centigrade  thermometer. 

Both  Eeaumur  and  Celsius  call  the  freezing  point  0°,  Fahren- 
heit calls  it  32°,  because  he  started  from  a  zero  which  he  incor- 
rectly imagined  was  the  greatest  terrestrial  cold.  Fahrenheit's 
boiling  point  is  therefore  212°.  Reaumur's  boiling  point  is  80°, 
while  the  boiling  point  of  Celsius  is  100°. 

The  length  of  the  degrees  being  in  the  proportion  of  80  :  100  : 
180,  or  of  4  :  5  :  9 ;  nothing  can  be  easier  than  to  convert  one  into 
the  other.  If  you  want  to  convert  Fahrenheit  into  Celsius,  mul- 
tiply by  5  and  divide  by  9 ;  if  Celsius  into  Fahrenheit,  multiply 
by  9  and  divide  by  5.  Thus  20°  of  Celsius  are  equal  to  36° 
Fahrenheit ;  but  if  we  would  know  what  temperature  by  Fah- 
renheit's thermometer  corresponds  to  20°  of  Celsius,  we  must 


CALOKIC   DOES   NOT   EXIST.  109 

add  32  to  the  36,  which  would  make  the  temperature  20°,  as 
shown  by  Celsius,  equal  the  temperature  68°,  as  shown  by  Fah- 
renheit. 


EXTEACTS  FEOM  SIR  II.  DAVY'S  FIRST  SCIENTIFIC  MEMOIR,  BEAR- 
ING THE  TITLE  '  ON  HEAT,  LIGHT,  AND  THE  COMBINATIONS  OF 
LIGHT.1  * 

THE  peculiar  modes  of  existence  of  bodies,  solidity,  fluidity, 
and  gazity,  depend  (according  to  the  calorists)  on  the  quantity 
of  the  fluid  of  heat  entering  into  their  composition.  This  sub- 
stance insinuating  itself  between  their  corpuscles,  separating  them 
from  each  other,  and  preventing  their  actual  contact,  is  by  them 
supposed  to  be  the  cause  of  repulsion. 

Other  philosophers,  dissatisfied  with  the  evidences  produced  in 
favour  of  the  existence  of  this  fluid,  and  perceiving  the  genera- 
tion of  heat  by  friction  and  percussion,  have  supposed  it  to  be  the 
motion.  Considering  the  discovery  of  the  true  cause  of  the  repul- 
sive power  as  highly  important  to  philosophy,  I  have  endeav- 
oured to  investigate  this  part  of  chemical  science  by  experiments ; 
from  these  experiments  (of  which  I  am  now  about  to  give  a  detail) 
I  conclude  that  heat  or  the  power  of  repulsion  is  not  matter. 

The  Phenomena  of  Repulsion  are  not  dependent  on,  a  peculiar  elastic 
fluid  for  their  existence,  or  Caloric  does  not  exist. 

Without  considering  the  effects  of  the  repulsive  power  on 
bodies,  or  endeavouring  to  prove  from  these  effects  that  it  is  mo- 
tion, I  shall  attempt  to  demonstrate  by  experiments,  that  it  is  not 
matter;  and  in  doing  this,  I  shall  use  the  method  called  by 
mathematicians,  reductio  ad  dbsurdum. 

First,  let  the  increase  of  temperature  produced  by  friction  and 
percussion  be  supposed  to  arise  from  a  diminution  of  the  capaci- 
ties of  the  acting  bodies.  In  this  case  it  is  evident  some  change 
must  be  induced  in  the  bodies  by  the  action,  which  lessens  their 
capacities  and  increases  their  temperatures. 

Experiment. — I  procured  two  parallelopipedons  of  icef,  of  the 

*  Sir  Humphry  Davy's  works,  vol.  ii. 

f  The  result  of  this  experiment  is  the  same,  if  wax,  tallow,  resin,  or 


110  APPENDIX   TO   LECTTJKE   III. 

temperature  of  29°,  six  inches  long,  two  wide,  and  two-thirds  of 
an  inch  thick ;  they  were  fastened  by  wires  to  two  bars  of  iron. 
By  a  peculiar  mechanism,  their  surfaces  were  placed  in  contact, 
and  kept  in  a  continued  and  most  violent  friction  for  some  min- 
utes. They  were  almost  entirely  converted  into  water,  which 
water  was  collected,  and  its  temperature  ascertained  to  be  35°, 
after  remaining  in  an  atmosphere  of  a  lower  temperature  for  some 
minutes.  The  fusion  took  place  only  at  the  plane  of  contact  of 
the  two  pieces  of  ice,  and  .no  bodies  were  in  friction  but  ice. 

From  this  experiment  it  is  evident  that  ice  by  friction  is  con- 
verted into  water,  and  according  to  the  supposition,  its  capacity 
is  diminished ;  but  it  is  a  well-known  fact,  that  the  capacity  of 
water  for  heat  is  much  greater  than  that  of  ice ;  and  ice  must  have 
an  absolute  quantity  of  heat  added  to  it,  before  it  can  be  convert- 
ed into  water.  Friction  consequently  does  not  diminish  the  ca- 
pacities of  bodies  for  heat. 

From  this  experiment  it  is  likewise  evident,  that  the  increase 
of  temperature  consequent  on  friction  cannot  arise  from  the  de- 
composition of  the  oxygen  gas  in  contact,  for  ice  has  no  attrac- 
tion for  oxygen.  Since  the  increase  of  temperature  consequent 
on  friction  cannot  arise  from  the  diminution  of  capacity,  or  oxy- 
dation  of  the  acting  bodies,  the  only  remaining  supposition  is, 
that  it  arises  from  an  absolute  quantity  of  heat  added  to  them, 
which  heat  must  be  attracted  from  the  bodies  in  contact.  Then 
friction  must  induce  some  change  in  bodies,  enabling  them  to  at- 
tract heat  from  the  bodies  in  contact. 

Experiment— -I  procured  a  piece  of  clockwork,  so  constructed 
as  to  be  set  at  work  in  the  exhausted  receiver ;  one  of  the  external 
wheels  of  this  machine  came  in  contact  with  a  thin  metallic  plate. 
A  considerable  degree  of  sensible  heat  was  produced  by  friction 
between  the  wheel  and  plate  when  the  machine  worked,  uninsu- 
lated from  bodies  capable  of  communicating  heat.  I  next  pro- 
cured a  small  piece  of  ice ;  *  round  the  superior  edge  of  this  a 

any  substance  fusible  at  a  low  temperature,  be  used ;  even  iron  may  be 
fused  by  collision. 

*  The  temperature  of  the  ice  and  of  the  surrounding  atmosphere  at 
the  commencement  of  the  experiment  was  32°,  that  of  the  machine  was 
likewise  33°.  At  the  end  of  the  experiment  the  temperature  of  the  coldest 


FUSION  OF  ICE  BY  FRICTION.  Ill 

small  canal  was  made,  and  filled  with  water.  The  machine  was 
placed  on  the  ice,  but  not  in  contact  with  the  water.  Thus  dis- 
posed, the  whole  was  placed  under  the  receiver  (which  had  been 
previously  filled  with  carbonic  acid),  a  quantity  of  potash  (i.  e. 
caustic  vegetable  alkali)  being  at  the  same  time  introduced. 

The  receiver  was  now  exhausted.  From  the  exhaustion  and 
from  the  attraction  of  the  carbonic  acid  gas  by  the  potash,  a  va- 
cuum nearly  perfect,  was,  I  believe,  made. 

The  machine  was  now  set  to  work ;  the  wax  rapidly  melted, 
proving  an  increase  of  temperature. 

Caloric  then  was  collected  by  friction ;  which  caloric,  on  the 
supposition,  was  communicated  by  the  bodies  in  contact  with  the 
machine.  In  this  experiment,  ice  was  the  only  body  in  contact 
with  the  machine.  Had  this  ice  given  out  caloric,  the  water  on 
the  top  of  it  must  have  been  frozen.  The  water  on  the  top  of  it 
was  not  frozen,  consequently  the  ice  did  not  give  out  caloric. 
The  caloric  could  not  come  from  the  bodies  in  contact  with  the 
ice,  for  it  must  have  passed  through  the  ice  to  penetrate  the  ma- 
chine, and  an  addition  of  caloric  to  the  ice  would  have  converted 
it  into  water. 

Heat,  when  produced  by  friction,  cannot  be  collected  from  the 
bodies  in  contact,  and  it  was  proved,  by  the  first  experiment,  that 
the  increase  of  temperature  consequent  on  friction  cannot  arise 
from  diminution  of  capacity  or  oxydation.  But  if  it  be  considered 
as  matter,  it  must  be  produced  in  one  of  these  modes.  Since  (as 
is  demonstrated  by  these  experiments)  it  is  produced  in  neither 
of  these  modes,  it  cannot  be  considered  as  matter.  It  has  there- 
fore been  experimentally  demonstrated  that  caloric,  or  the  matter 
of  heat,  does  not  exist. 

Solids,  by  long  and  violent  friction,  become  expanded,  and  if 
of  a  higher  temperature  than  our  bodies,  affect  the  sensory  organs 
with  the  peculiar  sensation  known  by  the  common  name  of  heat. 

part  of  the  machine  was  near  33°,  that  of  the  ice  and  surrounding  atmo- 
sphere the  same  as  at  the  commencement  of  the  experiment ;  so  that  the 
heat  produced  by  the  friction  of  the  different  parts  of  the  machine  was 
sufficient  to  raise  the  temperature  of  near  half  a  pound  of  metal  at  least 
one  degree ;  and  to  convert  eighteen  grains  of  wax  (the  quantity  employed) 
into  -a  fluid. 


112  APPENDIX   TO   LECTURE   III. 

Since  bodies  become  expanded  by  friction,  it  is  evident  that 
their  corpuscles  must  move  or  separate  from  each  other. 

Now  a  motion  or  vibration  of  the  corpuscles  of  bodies  must 
be  necessarily  generated  by  friction  and  percussion.  Therefore 
we  may  reasonably  conclude  that  this  motion  or  vibration  is  heat, 
or  the  repulsive  power. 

Heat,  then,  or  that  power  which  prevents  the  actual  contact 
of  the  corpuscles  of  bodies,  and  which  is  the  cause  of  our  peculiar 
sensations  of  heat  and  cold,  may  be  defined  a  peculiar  motion, 
probably  a  vibration  of  the  corpuscles  of  bodies,  tending  to 
separate  them.  It  may  with  propriety  be  called  the  repulsive 
motion. 

Since  there  exists  a  repulsive  motion,  the  particles  of  bodies 
may  be  considered  as  acted  on  by  two  opposing  forces ;  the  ap- 
proximating power,  which  may  (for  greater  ease  of  expression)  be 
called  attraction,  and  the  repulsive  motion.  The  first  of  these  is 
the  compound  effect  of  the  attraction  of  cohesion,  by  which  the 
particles  tend  to  come  in  contact  with  each  other ;  the  attraction 
of  gravitation,  by  which  they  tend  to  approximate  to  the  great 
contiguous  masses  of  matter,  and  the  pressure  under  which  they 
exist,  dependent  on  the  gravitation  of  the  superincumbent  bodies. 

The  second  is  the  effect  of  a  peculiar  motory  or  vibratory  im- 
pulse given  to  them,  tending  to  remove  them  farther  from  each 
other,  and  which  can  be  generated,  or  rather  increased,  by  friction 
or  percussion.  The  effects  of  the  attraction  of  cohesion,  the  great 
approximating  cause,  on  the  corpuscles  of  bodies,  is  exactly  simi- 
lar to  that"  of  the  attraction  of  gravitation  on  the  great  masses  of 
matter  composing  the  universe,  and  the  repulsive  force  is  analo- 
gous to  the  planetary  projectile  force. 

In  his  '  Chemical  Philosophy,'  pp.  94  and  95,  Davy  expresses 
himself  thus  : — '  By  a  moderate  degree  of  friction,  as  it  would 
appear  from  Eumford's  experiments,  the  same  piece  of  metal  may 
be  kept  hot  for  any  length  of  time ;  so  that,  if  the  heat  be  pressed 
out,  the  quantity  must  be  inexhaustible.  When  any  body  is 
cooled,  it  occupies  a  smaller  volume  than  before ;  it  is  evident, 
therefore,  that  its  parts  must  have  approached  each  other ;  when 
the  body  has  expanded  by  heat,  it  is  equally  evident  that  its 
parts  must  have  separated  from  each  other.  The  immediate  cause 
of  the  phenomenon  of  heat,  then,  is  motion,  and  the  laws  of  its 


DAVY  ON  THE  MOTION  OF  HEATJ         113 


communication  are  precisely  the  same  as  the  laws  of  the  commu- 
nication of  motion.' 

Since  all  matter  may  be  made  to  fill  a  smaller  space  by  cool- 
ing, it  is  evident  that  the  particles  of  matter  must  have  space  be- 
tween them ;  and  since  every  body  can  communicate  the  power 
of  expansion  to  a  body  of  a  lower  temperature — that  is,  can  give 
an  expansive  motion  to  its  particles — it  is  a  probable  inference 
that  its  own  particles  are  possessed  of  motion ;  but  as  there  is  no 
change  in  the  position  of  its  parts,  as  long  as  its  temperature  is 
uniform,  the  motion,  if  it  exist,  must  be  a  vibratory  or  undulatory 
motion,  or  a  motion  of  the  particles  round  their  axes,  or  a  motion 
of  the  particles  round  each  other. 

It  seems  possible  to  account  for  all  the  phenomena  of  heat,  if 
it  be  supposed  that  in  solids  the  particles  are  in  a  constant  state 
of  vibratory  motion,  the  particles  of  the  hottest  bodies  moving 
with  the  greatest  velocity,  and  through  the  greatest  space ;  that 
in  fluids  and  elastic  fluids,  besides  the  vibratory  motion,  which 
must  be  conceived  greatest  in  the  last,  the  particles  have  a  motion 
round  their  own  axes  with  different  velocity,  the  particles  of 
elastic  fluids  moving  with  the  greatest  quickness,  and  that  in 
ethereal  substances  the  particles  move  round  their  own  axes,  and 
separate  from  each  other,  penetrating  in  right  lines  through  space. 
Temperature  may  be  conceived  jto  depend  upon  the  velocity  of 
the  vibrations ;  increase  of  capacity  in  the  motion  being  performed 
in  greater  space ;  and  the  diminution  of  temperature  during  the 
conversion  of  solids  into  fluids  or  gases,  may  be  explained  on  the 
idea  of  the  loss  of  vibratory  motion,  in  consequence  of  the  revo- 
lution of  particles  round  their  axes,  at  the  moment  when  the  body 
becomes  fluid  or  aeriform,  or  from  the  loss  of  rapidity  of  vibration 
in  consequence  of  the  motion  of  the  particles  through  space. 


LECTUKE    IV. 

[February  13,  1862.] 

THE  TREYELYAN  INSTRUMENT — GORE'S  REYOLYING  BALLS — INFLUENCE  OF 
PRESSURE  ON  FUSING  POINT — LIQUEFACTION  AND  LAMINATION  OF  ICE  BY 
PRESSURE — DISSECTION  OF  ICE  BY  A  CALORIFIC  BEAM — LIQUID  FLOWERS 
AND  THEIR  CENTRAL  SPOT — MECHANICAL  PROPERTIES  OF  WATER  PURGED 
OF  AIR — THE  BOILING  POINT  OF  LIQUIDS  :  INFLUENCING  CIRCUMSTANCES 
—THE  GEYSERS  OF  ICELAND. 

APPENDIX  : — NOTE   ON   THE  TREYELYAN  INSTRUMENT — PHYSICAL  PROPERTIES 

OF   ICE. 

BEFORE-  finally  quitting  the  subject  of  expansion,  I 
wish  to  show  you  an  experiment  which  illustrates  in  a 
curious  and  agreeable  way  the  conversion  of  heat  into  me- 
chanical energy.  The  fact  which  I  wish  to  reproduce  was 
first  observed  by  a  gentleman  named  Schwartz,  in  one  of 
the  smelting  works  of  Saxony.  A  quantity  of  silver  which 
had  been  fused  in  a  ladle  was  allowed  to  solidify,  and  to 
hasten  its  cooling  it  was  turned  out  upon  an  anvil.  Some 
time  afterwards,  a  strange  buzzing  sound  was  heard  in  the 
locality,  and  was  finally  traced  to  the  hot  silver,  which  was 
found  quivering  upon  the  anvil.  Many  years  subsequent  to 
this,  Mr.  Arthur  Trevelyan  chanced  to  be  using  a  hot  sol- 
dering-iron, which  he  laid  by  accident  against  a  piece  of 
lead.  Soon  afterwards,  his  attention  was  excited  by  a 
most  singular  sound  which,  after  some  searching,  was  found 
to  proceed  from  the  soldering-iron.  Like  the  silver  of 


VIBRATIONS   OF   HEATED  METALS. 


115 


Schwartz,  the  soldering-iron  was  found  in  a  state  of  vibra- 
tion. Mr.  Trevelyan  made  his  discovery  the  subject  of  a 
very  interesting  investigation.  He  determined  the  best 
form  to  be  given  to  the  '  rocker '  as  the  vibrating  mass  is 
now  called,  and  throughout  Europe  at  present  this  instru- 
ment is  known  as  '  Trevelyan's  Instrument.'  Since  that 
time  the  subject  has  engaged  the  attention  of  Prof.  J.  D. 
Forbes,  Dr.  Seebeck,  Mr.  Faraday,  M.  Sondhaus,  and  my- 
self; but  to  Trevelyan  and  Seebeck  we  owe  most. 

Here  is  such  a  rocker  made  of  brass.  Its  length,  A  c 
(fig.  26),  is  five  inches,  the  width  A  B,  1*5  in.,  and  the  length 
of  the  handle,  which  terminates  in  the  knob  r,  is  ten  inches. 


Fig.  26. 


<£> 


Fi?.  27. 


A  groove  runs  at  the  back  of  the  rocker,  along  its  centre  ; 
the  cross  section  of  the  rocker  and  its  groove  is  given  at  M. 
I  heat  the  rocker  to  a  temperature  somewhat  higher  than 
that  of  boiling  water,  and  lay  it  on  this  block  of  lead,  al- 
lowing its  knob 
to  rest  upon  the 
table.  You  hear 
a  quick  succes- 
sion of  forcible 
taps.  But  you 
cannot  see  the 
oscillations  of 
the  rocker  to 
wThich  the  taps 
are  due.  I 
therefore  place 
on  it  this  rod 
of  brass,  A  B  (fig.  27),  with  two  balls  of  brass  at  its  end, 


116  LECTURE   IV. 

the  oscillations  are  thereby  rendered  much  slower,  and  you 
can  easily  follow  with  the  eye  the  pendulous  motion  of  the 
rod  and  balls.  This  motion  will  continue  as  long  as  the 
rocker  is  able  to  communicate  sufficient  heat  to  the  carrier 
on  which  it  rests.  Thus  w^e  render  the  vibrations  slow,  but 
I  can  also  render  them  quick  by  using  a  rocker  with  a 
wider  groove.  The  sides  of  this  rocker  do  not  overhang  so 
much  as  those  of  the  last ;  it  is  virtually  a  shorter  pendu- 
lum, and  will  vibrate  more  quickly.  Placed  upon  the  lead, 
as  before,  it  commences  an  unsteady  and  not  altogether 
pleasant  music.  It  is  still  restless,  sometimes  seeming  to 
expostulate,  sometimes  even  to  objurgate,  as  if  it  disliked 
the  treatment  to  which  it  is  subjected.  Now  it  becomes 
mellow,  and  fills  the  room  with  a  clear  full  note.  Its  taps 
have  become  periodic  and  regular,  and  have  linked  them- 
selves together  to  produce  music.  Here  is  a  third  rocker, 
with  a  still  wider  groove,  and  with  it  I  can  obtain  a  shriller 
tone.  You  know  of  course  that  the  pitch  of  note  augments 
with  the  number  of  the  vibrations;  this  wide-grooved 
rocker  oscillates  more  quickly,  and  therefore  emits  a  higher 
note.  By  casting  a  beam  of  light  upon  the  rocker  I  obtain 
a  better  index  than  the  rod  and  balls.  This  index  is  with- 
out weight,  and  therefore  does  not  retard  the  motion  of  the 
rocker.  To  the  latter  I  have  fastened,  by  a  single  screw 
at  its  centre,  a  small  disk  of  polished  silver  ;  on  which  the 
beam  of  the  electric  lamp  now  falls,  and  is  reflected  against 
the  screen.  When  the  rocker  Vibrates,  the  beam  vibrates 
also,  but  with  twice  the  angular  velocity,  and  there  you  see 
the  patch  of  light  quivering  upon  the  screen. 

What  is  the  cause  of  these  singular  vibrations  and 
tones  ?  They  are  due  simply  to  the  sudden  expansion  by 
heat  of  the  body  on  which  the  rocker  rests.  Whenever  the 
hot  rocker  comes  into  contact  with  its  lead  carrier,  a  nipple 
suddenly  juts  from  the  latter,  being  produced  by  the  heat 
communicated  to  the  lead  at  the  point  of  contact.  The 


THE   TEEVELYAN   INSTRUMENT. 


117 


rocker  is  tilted  up,  and  some  other  point  of  it  comes  into 
contact  with  the  lead,  a  fresh  nipple  is  produced,  and  the 
rocker  is  again  tilted.  Let  A  B  (fig.  28)  be  the  surface  of 
the  lead,  and  K  the  cross  section  of  the  hot  rocker ;  tilted 
to  the  right,  the  nipple  is  formed  as  at  K  ;  tilted  to  the  left, 
it  is  formed  as  at  L.  The  consequence  is  that  until  its  tern- 
rig.  23. 


perature  falls  sufficiently,  the  rocker  is  tossed  to  and  fro, 
and  the  quick  succession  of  its  taps  against  the  lead  pro- 
duces a  musical  sound. 

I  have  here  fixed  two  pieces  of  sheet  lead  in  a  vice ; 
their  edges  are  exposed,  and  are  about  half  an  inch  asun- 
der. I  balance  a  long  bar  of  heated  brass  across  the  two 
lead  edges.  It  rests  first  on  one  edge,  which  expands  at 
the  point  of  contact  and  jerks  it  upwards  ;  it  then  falls  upon 
the  second  edge  which  also  rejects  it ;  and  thus  it  goes  on 
oscillating,  and  will  continue  to  do  so  as  long  as  the  bar 

Fig.  29. 


can  communicate  sufficient  heat  to  the  lead.   This  fire-shovel 
will  answer  quite  as  well  as  the  prepared  bar.     I  balance 


118  •    LECTURE  IV. 

the  heated  shovel  thus  upon  the  edges  of  the  lead,  and  it 
oscillates  exactly  as  the  bar  did  (fig.  29).  I  may  add,  that 
by  properly  laying  either  the  poker  or  the  fire-shovel  upon 
a  block  of  lead,  supporting  the  handle  so  as  to  avoid  fric- 
tion, you  may  obtain  notes  as  sweet  and  musical  as  any 
which  you  have  heard  to-day.  A  heated  hoop  placed 
upon  a  plate  of  lead  may  be  caused  to  vibrate  and  sing ; 
and  a  hot  penny-piece  or  half  crown  may  be  caused  to 
do  the  same.* 

Looked  at  with  an  eye  to  the  connection  of  natural 
forces,  this  experiment  is  interesting.  The  atoms  of  bodies 
must  be  regarded  as  all  but  infinitely  small,  but  then  they 
must  be  regarded  as  all  but  infinitely  numerous.  The  aug- 
mentation of  the  amplitude  of  any  oscillating  atom  by  the 
communication  of  heat,  is  insensible,  but  the  summation 
of  an  almost  infinite  number  of  such  augmentations  become 
sensible.  Such  a  summation,  effected  almost  in  an  in- 
stant, produces  our  nipple,  and  tilts  the  heavy  mass  of  the 
rocker.  Here  we  have  a  direct  conversion  of  heat  into 
common  mechanical  motion.  But  the  tilted  rocker  falls 
again  by  gravity,  and  in  its  collision  with  the  block  restores 
almost  the  precise  amount  of  heat  which  was  consumed  in 
lifting  it.  Here  we  have  the  direct  conversion  of  common 
gravitating  force  into  heat.  Again  the  rocker  is  surrounded 
by  a  medium  capable  of  being  set  in  motion.  The  air  of 
this  room  weighs  some  tons,  and  every  particle  of  it  is 
shaken  by  the  rocker,  and  every  tympanic  membrane,  and 
every  auditory  nerve  present,  is  similarly  shaken.  Thus  we 
have  the  conversion  of  a  portion  of  heat  into  sound.  And, 
finally,  every  sonorous  vibration  which  speeds  through  the 
air  of  this  room,  and  wastes  itself  upon  the  walls,  seats, 
and  cushions,  is  converted  into  the  form  with  which  the 
cycle  of  actions  commenced — namely,  into  heat. 

*  For  further  information  see  Appendix  to  this  lecture. 


ROTATION   BY   ELECTRICITY.  119 

Jlere  is  another  curious  effect,  for  which  we  are  indebt- 
ed to  Mr.  George  Gore,  and  which  admits  of  a  similar  ex- 
planation. You  see  this  line  of  rails.  Two  strips  of  brass, 
s  s,  s'  s'  (fig.  30),  are  set  edgeways,  and  about  an  inch 
asunder.  I  place  this  hollow  metal  ball  B  upon  the  rails  ; 
if  I  push  it,  it  rolls  along  them;  but  if  I  do  not  push 
it,  it  stands  still.  I  connect  these  two  rails,  by  the  wires 


Fig.  80. 


w  w',  with  the  two  poles  of  a  Voltaic  battery.  A  current 
now  passes  down  one  rail  to  the  metal  ball,  thence  along 
the  ball  to  the  other  rail,  and  finally  back  to  the  battery. 
In  passing  from  the  rail  to  the  ball,  and  from  the  ball  to 
the  other  rail,  the  current  encounters  resistance,  and  where- 
ever  a  current  encounters  resistance,  heat  is  developed. 
Heat,  therefore,  is  generated  at  the  two  points  of  contact 
of  the  ball  with  the  rails  ;  and  this  heat  produces  an  eleva- 
tion of  the  rail  at  these  points.  Observe  the  effect ;  the 
ball  which  a  moment  ago  was  tranquil  is  now  very  uneasy. 
It  vibrates  a  little  at  first  without  rolling  ;  now  it  actually 
rolls  a  little  way,  stops,  and  rolls  back  again.  It  gradually 
augments  its  excursion,  now  it  has  gone  further  than  I 
intended:  it  has  quite  rolled  off  the  rails,  and  injured  itself 
by  falling  on  the  floor. 

Here  is  another  apparatus  for  which  I  am  indebted  to 
Mr.  Gore  himself,  and  in  which  the  rails  form  a  pair  of 
concentric  hoops  ;  when  the  circuit  is  established,  the  ball 
F  (fig.  31)  rolls  round  the  circle.*  Mr.  Gore  has  also  ob- 
tained the  rotation  of  light  balls,  by  placing  them  on  cir- 

*  Phil.  Mag.,  vol.  15,  p.  521. 


120 


LECTURE  III. 


cular  rails  of  hot  copper,  the  rolling  force  in  this  case 
being  the  same  as  the  rocking  force  in  the  Trevelyan  in- 
strument. 

In  my  last  lecture  I  made  evident  to  you  the  expansion 
of  water  when  it  passes  from  the  liquid  to  the  solid  condi- 
tion ;  with  most  other  substances  solidification  is  accom- 

Fig.  31. 


panied  by  contraction.  I  have  here  a  round  glass  dish  into 
which  I  pour  some  hot  water.  Over  the  water  I  pour  from 
a  ladle  a  quantity  of  melted  bees'-wax.  The  wax  now 
forms  a  liquid  layer  nearly  half  an  inch  thick  above  the 
water.  We  will  suffer  both  water  and  wax  to  cool,  and 
when  they  are  cool  you  will  find  that  the  wax  which  now 
overspreads  the  entire  surface,  and  is  attached  all  round  to 
the  glass,  will  retreat,  and  we  shall  finally  obtain  a  cake  of 
wax  of  considerably  smaller  area  than  the  dish. 

The  wax,  then,  in  passing  from  the  solid  to  the  liquid 
state  expands.  To  assume  the  liquid  form,  its  particles 
must  be  pushed  more  widely  apart,  a  certain  play  between 
the  particles  being  necessary  to  the  condition  of  liquidity. 
Now  supposing  we  resist  the  expansion  of  the  wax  by  an 
external  mechanical  force  ;  suppose  we  have  a  very  strong 
vessel  completely  filled  with  solid  wax,  and  which  offers  a 
powerful  resistance  to  the  expansion  of  the  mass  within  it ; 


INFLUENCE  OF  PRESSURE  ON  FUSING  POINT.         121 

what  would  you  expect  if  you  sought  to  liquefy  the  wax  in 
this  vessel  ?  When  the  wax  is  free,  the  heat  has  only  to 
conquer  the  attraction  of  its  own  particles,  but  in  the  strong 
vessel  it  has  not  only  to  conquer  the  attraction  of  the  par- 
ticles, but  also  the  resistance  offered  by  the  vessel.  By  a  mere 
process  of  reasoning,  we  should  thus  be  led  to  infer  that  a 
greater  amount  of  heat  would  be  required  to  melt  the  wax 
under  pressure,  than  when  it  is  free ;  or,  in  other  words, 
that  the  point  of  fusion  of  the  wax  is  elevated  by  pressure. 
This  reasoning  is  completely  justified  by  experiment,  not 
only  with  wax,  but  with  other  substances  which  contract  on 
solidifying,  and  expand  on  liquefying.  Messrs.  Hopkins 
and  Fairbairn  have,  by  pressure,  raised  the  melting  point 
of  some  substances  which  contract  considerably  on  solidify- 
ing as  much  as  20°  and  30°  Fahr. 

These  experiments  bear  on  a  very  remarkable  specula- 
tion. The  earth  is  known  gradually  to  augment  in  temper- 
ature as  we  pierce  it  deeper,  and  the  depth  has  been  calcu- 
lated at  which  all  known  terrestrial  bodies  would  be  in  a 
state  of  fusion.  Mr.  Hopkins,  however,  observes  that 
owing  to  the  enormous  pressure  of  the  superincumbent 
layers,  the  deeper  strata  would  require  a  far  higher  tem- 
perature to  fuse  them,  than  would  be  necessary  to  fuse  the 
strata  near  the  earth's  surface.  Hence  he  infers  that  the 
solid  crust  must  have  a  considerably  greater  thickness  than 
that  given  by  a  calculation,  which  assumed  the  fusing 
points  of  the  superficial  and  the  deeper  strata  to  be  the 
same. 

Now  let  us  turn  from  wax  to  ice.  Ice,  on  liquefying, 
contracts  •  in  the  arrangement  of  its  atoms  to  form  a  solid, 
more  room  is  required  than  they  need  in  the  neighbouring 
liquid  state.  No  doubt  this  is  due  to  crystalline  arrange- 
ment ;  the  attracting  poles  of  the  molecules  are  so  placed 
that  when  the  crystallising  force  comes  into  play,  the  mole- 
ctales  unite  so  as  to  leave  larger  inter-atomic  spaces  in  the 


122  LECTUKE  IV. 

mass.  We  may  suppose  them  to  attach  themselves  by  their 
corners  ;  and  in  turning  corner  to  corner,  to  cause  a  reces- 
sion of  the  atomic  centres.  At  all  events  their  centres  re- 
treat from  each  other  when  solidification  sets  in.  By  cool- 
ing, then,  this  power  of  retreat,  and  of  consequent  enlarge- 
ment of  volume,  is  conferred.  It  is  evident  that  pressure 
in  this  case  would  resist  the  expansion  which  is  necessary 
to  solidification,  and  hence  the  tendency  of  pressure,  in  the 
case  of  water,  is  to  keep  it  liquid.  Thus  reasoning,  we 
should  be  led  to  the  conclusion  that  the  fusing  points  of 
substances  which  expand  on  solidifying  are  lowered  by 
pressure. 

Professor  James  Thomson  first  drew  attention  to  this 
fact,  and  his  theoretic  reasonings  have  been  verified  by  the 
experiments  of  his  brother  Professor  William  Thomson. 

Let  us  illustrate  these  principles  by  a  striking  experi- 
ment. I  have  here  a  square  pillar  of  clear  ice  an  inch  and 
a  half  in  height  and  about  a  square  inch  in  cross  section. 
At  present  the  temperature  of  this  ice  is  0°  C.  But  sup- 
pose I  subject  this  ice  to  pressure,  I  lower  its  point  of  fu- 
sion :  the  ice  under  pressure  will  melt  at  a  temperature 
under  0°  C.,  and  hence  the  temperature  which  it  now  pos- 
sesses is  in  excess  of  that  at  which  it  will  melt  under  pres- 
sure. I  have  cut  this  ice  so  that  its  planes  of  freezing  are 
perpendicular  to  the  height  of  the  pillar.  The  direction  of 
the  stratified  air-bubbles  in  the  ice  from  which  this  clear 
piece  was  taken,  enabled  me  to  fix  at  once  upon  its  planes 
of  freezing.  Well,  I  place  the  column  of  ice,  L,  upright 
between  two  slabs  of  boxwood,  B  B'  (fig.  32),  and  place  the 
whole  between  the  plates  of  this  small  hydraulic  press ; 
through  the  ice  I  send  a  beam  from  the  electric  lamp.  In 
front  of  the  ice  I  place  a  lens,  and  by  it  project  a  magnified 
image  of  the  ice  upon  the  screen  before  you.  The  beam 
which  passes  through  the  ice  has  been  purified  beforehand, 
so  that,  although  it  is  still  hot,  its  heat  is  not  of  such  a 


LIQUEFACTION   OF  ICE  BY   PRESSURE. 


123 


quality  as  can  melt  the  ice  ;  hence  the  light  passes  through 
the  substance  without  causing  fusion.  I  work  the  arm  of 
the  press  ;  the  pillar  of  ice  is  now  gently  squeezed  between 
the  two  slabs  of  boxwood.  I  apply  the  pressure  cautiously, 
and  now  you  see  dark  streaks  beginning  to  show  them- 
selves across  the  ice,  at  right  angles  to  the  direction  of 
pressure.  Right  in  the  middle  of  the  mass  they  are  ap- 

Fig.  32. 


ig ;  and  as  I  continue  the  pressure,  the  old  streaks 
expand  and  new  ones  appear.  The  entire  column  is  now 
scarred  across  by  these  striae.  What  are  they  ?  They  are 
simply  liquid  layers  foreshortened,  and  when  you  examine 
this  column  and  look  into  it  obliquely,  you  see  these  sur- 
faces. We  have  liquefied  the  ice  in  planes  perpendicular  to 
the  pressure,  and  these  liquid  planes  interspersed  through- 
out the  mass  give  it  this  strongly  pronounced  laminated 
appearance.* 

Whether  as  a  solid,  a  liquid,  or  a  gas,  water  is  one  of 

*  See  Appendix  to  this  lecture  for  further  information. 


124:  LECTURE   IY. 

the  most  wonderful  substances  in  nature.  Let  us  consider 
its  wonders  a  little  further.  At  all  temperatures  above  32° 
Fahr.  or  0°  C.,  the  motion  of  heat  is  sufficient  to  keep  the 
molecules  of  water  from  rigid  union.  But  at  0°  C.  the  mo- 
tion becomes  so  reduced  that  the  atoms  then  seize  upon 
each  other  and  aggregate  to  a  solid.  This  union,  however, 
is  a  union  according  to  law.  To  many  persons  here  present 
this  block  of  ice  may  seem  of  no  more  interest  and  beauty 
than  a  block  of  glass  ;  but  in  the  estimation  of  science  it 
bears  the  same  relation  to  glass,  that  an  oratorio  of  Handel 
does  to  the  cries  of  a  market-place.  The  ice  is  music,  the 
glass  is  noise  ;  the  ice  is  order,  the  glass  is  confusion.  In 
the  glass,  molecular  forces  constitute  an  inextricably  entan- 
gled skein ;  in  the  ice  they  are  woven  to  a  symmetric  web ; 
the  miraculous  texture  of  which  I~will  now  try  to  reveal. 

How  shall  I  dissect  this  ice  ?  In  the  solar  beam, — or, 
failing  that,  in  the  beam  of  an  electric  lamp,  we  have  an 
anatomist  competent  to  perform  this  work.  I  remove  the 
agent  by  which  this  beam  was  purified  in  the  last  experi- 
ment, and  will  send  the  rays  direct  from  the  lamp  through 
this  slab  of  pellucid  ice.  It  shall  pull  the  crystal  edifice  to 
pieces  by  accurately  reversing  the  order  of  its  architecture. 
Silently  and  symmetrically  the  crystallizing  force  builds  the 
atoms  up,  silently  and  symmetrically  the  electric  beam  will 
take  them  down.  I  .place  this  slab  of  ice  in  front  of  the 
lamp,  the  light  of  which  now  passes  through  the  ice.  Com- 
pare the  beam  before  it  enters  with  the  beam  after  its  pass- 
age through  the  substance  :  to  the  eye  there  is  no  sensible 
difference  ;  the  light  is  scarcely  diminished.  Not  so  with 
the  heat.  As  a  thermic  agent,  the  beam,  before  entering, 
is  far  more  powerful  than  it  is  after  its  emergence.  A  por- 
tion of  the  beam  has  been  arrested  in  the  ice,  and  that  por- 
tion is  our  working  anatomist.  Well,  what  is  he  doing  ? 
I  place  a  lens  in  front  of  the  ice,  and  cast  a  magnified  image 


DISSECTION  OF   ICE.  125 

of  the  slab  upon  the  screen.  Observe  that  image  (fig.  33), 
which,  in  beauty,  falls  far  short  of  the  actual  effect.  Here 
we  have  a  star  and  there  a  star ;  and  as  the  action  contin- 
ues, the  ice  appears  to  resolve  itself  into  stars,  each  one 
possessing  six  rays,  each  one  resembling  a  beautiful  flower 
of  six  petals.  And  as  I  shift  my  lens  to  and  fro,  I  bring 
new  stars  into  view,  and  as  the  action  continues,  the  edges 
of  the  petals  become  serrated,  and  spread  themselves  out 
like  fern  leaves  upon  the  screen.  Probably  few  here  pres- 
ent were  aware  of  the  beauty  latent  in  a  block  of  common 
ice.  And  only  think  of  lavish  Nature  operating  thus 
throughout  the  world.  Every  atom  of  the  solid  ice  which 
sheets  the  frozen  lakes  of  the  North  has  been  fixed  accord- 
ing to  this  law.  Nature  4  lays  her  beams  in  music,'  and  it 
is  the  function  of  science  to  purify  our  organs,  so  as  to 
enable  us  to  hear  the  strain. 

And  now  I  have  to  draw  your  attention  to  two  points 
connected  with  this  experiment,  of  great  minuteness,  but 
of  great  interest.  You  see  these  flowers  by  transmitted 
light — by  the  light  which  has  passed  through  both  the 
flowers  and  the  ice.  But  when  you  examine  them,  by  al- 
lowing a  beam  to  fall  upon  them  and  to  be  reflected  from 
them  to  your  eye,  you  find  in  the  centre  of  each  flower  a 
spot  which  shines  with  the  lustre  of  burnished  silver.  You 
might  be  disposed  to  think  this  spot  a  bubble  of  air ;  but 
you  can,  by  immersing  it  in  hot  water,  melt  away  the  ice 
all  round  the  spot ;  and  the  moment  the  spot  is  thus  laid 
bare,  it  collapses,  and  no  trace  of  a  bubble  of  air  is  to  be 
seen.  The  spot  is  a  vacuu?n.  Observe  how  truly  Nature 
works ;  observe  how  rigidly  she  carries  her  laws  into  all 
her  operations.  We  learned  in  the  last  lecture,  that  ice  in 
melting  contracted,  and  here  we  find  the  fact  turning  up. 
The  water  of  these  flowers  cannot  fill  the  space  occupied 
by  the  ice  by  whose  fusion  they  are  produced,  hence  the 


LIQUID  FLOWERS   AND  CENTRAL   SPOT.  127 

production  of  a  vacuum  necessarily  accompanies  the  forma- 
tion of  every  liquid  flower. 

When  I  first  observed  these  beautiful  figures,  I  thought 
at  the  moment  when  the  central  spot  appeared,  like  a  point 
of  light  suddenly  formed  within  the  ice,  that  I  heard  a 
clink,  as  if  the  ice  had  split  asunder  when  the  bright  spot 
was  formed.  At  first  I  suspected  that  it  was  my  imagina- 
tion which  associated  sound  with  the  appearance  of  the 
spot,  as  it  is  said  that  people  who  see  meteors  often  ima- 
gine a  rushing  noise  when  they  really  hear  none.  The 
clink,  however,  was  a  reality ;  and  if  you  will  allow  me,  I 
will  now  make  this  trivial  fact  a  starting  point  from  which 
I  will  conduct  you  through  a  series  of  interesting  phenom- 
ena, to  a  far-distant  question  of  practical  science. 

All  water  holds  a  large  quantity  of  air  within  it  in  a 
state  of  solution ;  by  boiling  you  may  liberate  this  impris- 
oned air.  On  heating  a  flask  of  water  you  see  air  bubbles 
crowding  on  its  sides  long  before  it  boils,  and  you  see  the 
bubbles  rising  through  the  liquid  without  condensation, 
and  often  floating  on  the  top.  One  of  the  most  remarkable 
effects  of  this  air  in  the  water  is,  that  it  promotes  the  ebul-- 
lition  of  the  liquid.  It  acts  as  a  kind  of  elastic  spring, 
pushing  the  atoms  of  the  water  apart,  and  thus  helping 
them  to  take  the  gaseous  form. 

Now  suppose  this  air  removed  ;  having  lost  the  cushion 
which  separated  them,  the  atoms  lock  themselves  together 
in  a  far  tighter  embrace.  The  cohesion  of  the  water  is 
vastly  augmented  by  the  removal  of  the  air.  Here  is  a 
glass  vessel,  the  so-called  water  hammer,  which  contains 
water  purged  of  air.  One  effect  of  the  withdrawal  of  the 
elastic  buffer  is,  that  the  water  here  falls  with  the  sound 
of  a  solid  body.  You  hear  how  the  liquid  rings  against 
the  end  of  the  tube  when  I  turn  it  upside  down.  Here  is 
another  tube,  ABC  (fig.  34),  bent  into  the  form  of  a  V,  and 
intended  to  show  how  the  cohesion  of  the  water  is  affected 


128 


LECTUKE  IV. 


by  long  boiling.  I  bring  this  water  into  one  arm  of  tli* 
V ;  by  tilting  the  tube  it  flows,  as  you,  see,  freely  into  the 
other  arm.  I  restore  it  to  the  first  arm,  and  now  tap  the 
end  of  this  arm  against  the  table.  You  hear,  at  first,  a 
loose  and  jingling  sound.  As  long  as  you  hear  it  the  wa- 
ter is  not  in  true  contact  with  the  surface  of  the  tube.  I 

Fig.  34. 


continue  my  tapping :  you  mark  an  alteration  in  the  sound ; 
the  jingling  has  disappeared,  and  the  sound  is  now  hard, 
like  that  of  solid  against  solid.  I  now  raise,  my  tube.  Ob- 
serve what  occurs.  I  turn  the  column  of  water  upside 
down,  but  there  it  stands  in  A  B.  Its  particles  cling  so  te- 
naciously to  the  sides  of  the  tube,  and  lock  themselves  so 
firmly  together,  that  it  refuses  to  behave  like  a  liquid 
body ;  it  declines  to  obey  the  law  of  gravity. 

So  much  for  the  augmentation  of  cohesion ;  but  this 
very  cohesion  enables  the  liquid  to  resist  ebullition.  Wa- 
ter thus  freed  of  its  air  can  be  raised  to  a  temperature  100° 


DEPORTMENT  OF  WATER  PURGED  OF  AIR.     129 

and  more  above  its  ordinary  boiling  point,  without  ebulli- 
tion. But  mark  what  takes  place  when  the  liquid  does 
boil.  It  has  an  enormous  excess  of  heat  stored  up  ;  the 
locked  atoms  finally  part  company,  but  they  do  so  with  the 
violence  of  a  spring  which  suddenly  breaks  under  strong 
tension,  and  ebullition  is  converted  into  explosion.  For 
the  discovery  of  this  interesting  property  of  water  we  are 
indebted  to  M.  Donny,  of  Ghent. 

Turn  we  now  to  our  ice : — Water,  in  freezing,  complete- 
ly excludes  the  air  from  its  crystalline  architecture.  All 
foreign  bodies  are  squeezed  out  in  the  act  of  freezing,  and 
ice  holds  no  air  in  solution.  Supposing  then  that  we  melt 
a  piece  of  pure  ice  under  conditions  where  air  cannot  ap- 
proach it,  we  have  water  in  its  most  highly  cohesive  condi- 
tion ;  and  such  water  ought,  if  heated,  to  show  the  effects  to 
which  I  have  referred.  That  it  does  so  has  been  proved 
by  Mr.  Faraday.  He  melted  pure,  ice  under  spirit  of  tur- 
pentine, and  found  that  the  liquid  thus  formed  could  be 
heated  far  beyond  its  boiling  point,  and  that  the  rupture 
of  the  liquid,  by  the  act  of  ebullition,  took  place  with  al- 
most explosive  violence.  And  now,  let  us  apply  these  facts 
to  the  six-petaled  ice-flowers  and  their  little  central  star. 
They  are  formed  in  a  place  where  no  air  can  come.  Imag- 
ine the  flower  forming  and  gradually  augmenting  in  size. 
The  cohesion  of  the  liquid  is  so  great,  that  it  will  pull  the 
walls  of  its  chamber  together,  or  even  expand  its  own  vol- 
ume, sooner  than  give  way.  But  as  its  size  augments,  the 
space  which  it  tries  to  occupy  becomes  too  large  for  it, 
until  finally  the  liquid  snaps,  a  vacuum  is  formed,  and  a 
clink  is  heard. 

Let  us  now  take  our  final  glance  at  this  web  of  rela- 
tions. It  is  very  remarkable  that  a  great  number  of  loco- 
motives have  exploded  on  quitting  the  shed  where  they 
had  remained  for  a  time  quiescent.  The  number  of  explo- 
sions which  have  occurred  just  as  the  engineer  turned  on 
6* 


130  LECTURE   IV. 

the  steam  is  quite  surprising.  Now  supposing  that  a  loco- 
motive had  been  boiling  sufficiently  long  to  expel  the  air 
contained  in  its  water ;  that  liquid  would  possess,  in  a 
greater  or  less  degree,  the  high  cohesive  quality  to  which  I 
have  drawn  your  attention.  It  is  at  least  conceivable  that 
while  resting  previous  to  starting  on  its  journey,  an  excess 
of  heat  might  be  thus  stored  up  in  the  boiler,  and  if  stored 
up,  the  certain  result  would  be,  that  the  engineer  on  turn- 
ing on  the  steam  would,  by  a  mechanical  act,  produce  the 
rupture  of  the  cohesion,  and  steam  of  explosive  force  would 
instantly  be  generated.  I  do  not  say  that  this  is  the  case  ; 
but  who  can  say  that  it  is  not  the  case.  We  have  been 
dealing  throughout  with  a  real  agency,  which  is  certainly 
competent,  if  its  power  be  invoked,  to  produce  the  most 
terrible  effects. 

We  have  here  touched  on  the  subject  of  steam  ;  let  us 
bestow  a  few  minutes'  further  consideration  on  its  forma- 
tion and  action.  As  you  add  heat,  or  in  other  words,  mo- 
tion, to  water,  the  particles  from  its  free  surface  fly  off  in 
augmented  numbers.  We  at  length  approach  what  is  called 
the  boiling  point  of  the  liquid,  where  the  conversion  into 
vapour  is  not  confined  to  the  free  surface,  but  is  most  co- 
pious at  the  bottom  of  the  vessel  to  which  the  heat  is  ap- 
plied. When  water  boils  in  a  glass  beaker,  the  steam  is 
seen  rising  in  spheres  from  the  bottom  to  the  top,  where  it 
often  swims  for  a  time,  enclosed  above  by  a  dome-shaped 
liquid  film.  Now,  to  produce  these  bubbles,  certain  resist- 
ances must  be  overcome.  First,  we  have  the  adhesion  of 
the  water  to  the  vessel  which  contains  it,  and  this  force 
varies  with  the  substance  of  the  vessel.  In  the  case  of  a 
glass  vessel,  for  example,  the  boiling  point  may  be  raised 
two  or  three  degrees  by  adhesion ;  while  in  metal  vessels 
this  is  impossible.  The  adhesion  is  overcome  by  fits  and 
starts,  which  may  be  so  augmented  by  the  introduction  of 
salts  into  the  liquid,  that  a  loud  bumping  sound  accompa- 


BOILING    POINTS   OF    LIQUIDS.  131 

nies  the  ebullition ;  the  detachment  is  in  some  cases  so  sud- 
den and  violent  as  to  cause  the  liquid  to  jump  bodily  out 
of  the  vessel. 

A  second  antagonism  to  the  boiling  of  the  liquid  is  the 
attraction  of  the  liquid  particles  for  each  other,  a  force 
which,  as  we  have  seen,  may  become  very  powerful  when 
the  liquid  is  purged  of  air.  This  is  not  only  true  of  water, 
but  of  other  liquids — of  all  the  ethers  and  alcohols,  for 
example.  If  we  connect  a  small  flask  containing  ether  or 
alcohol  with  an  air  pump,  a  violent  ebullition  occurs  in  the 
liquid  when  the  pump  is  first  worked ;  but  after  all  the  air 
has  been  removed,  we  may,  in  many  cases,  continue  to  work 
the  pump,  without  producing  any  sensible  ebullition ;  the 
free  surface  alone  of  the  liquid  yielding  vapour. 

But  that  steam  should  exist  in  bubbles,  in  the  interior 
of  a  mass  of  liquid,  it  must  be  able  to  resist  two  other 
things,  the  weight  of  the  water  above  it,  and  the  weight 
of  the  atmosphere  above  the  water.  What  the  atmosphere 
is  competent  to  do  may  be  thus  illustrated.  I  have  here  a 
tin  vessel  containing  a  little  water,  which  is  kept  boiling  by 
this  small  lamp.  At  the  present  moment  all  the  space  above 
the  water  is  filled  with  steam,  which  issues  from  this  stop- 
cock. I  shut  off  the  cock,  withdraw  the  lamp,  and  pour 
cold  water  upon  the  tin  vessel.  The  steam  within  it  is  con- 
densed, the  elastic  cushion  which  pushed  the  sides  outwards 
in  opposition  to  the  pressure  of  the  atmosphere  is  withdrawn, 
and  observe  the  consequence.  The  sides  of  the  vessel  are 
crushed  and  crumpled  up  by  the  atmospheric  pressure. 
This  pressure  amounts  to  15  Ib.  on  every  square  inch :  how 
then,  can  a  thing  so  frail  as  a  bubble  of  steam  exist  on  the 
surface  of  boiling  water  ?  simply  because  the  elastic  force 
of  the  steam  within  is  exactly  equal  to  that  of  the  atmos- 
phere without ;  the  liquid  film  is  pressed  between  two  elas- 
tic cushions  which  exactly  neutralize  each  other.  If  the 
steam  were  predominant,  the  bubble  would  burst  from 


132  LECTUKE  IV. 

within  outwards ;  if  the  air  were  predominant,  the  bubble 
would  be  crushed  inwards.  Here,  then,  we  have  the  true 
definition  of  the  boiling  point  of  a  liquid.  It  is  that  tem- 
perature at  which  the  tension  of  its  vapour  exactly  balances 
the  pressure  of  the  atmosphere. 

As  we  ascend  a  mountain  the  pressure  of  the  atmos- 
phere above  us  diminishes,  and  the  boiling  point  is  corres- 
pondingly lowered.  On  an  August  morning  in  1859  I 
found  the  temperature  of  boiling  water  on  the  summit  of 
Mont  Blanc  to  be  184'95°  Fahr. ;  that  is,  about  27°  lower 
than  the  boiling  point  at  the  sea  level.  On  August  3, 1858, 
the  temperature  of  boiling  water  on  the  summit  of  the 
Finsteraarhorn  was  187°  Fahr.  On  August  10,  1858,  the 
boiling  point  on  the  summit  of  Monte  Rosa  wTas  184*92° 
Fahr.  The  boiling  point  on  Monte  Rosa  is  shown  by  these 
observations  to  be  almost  the  same  as  it  was  found  to  be 
on  Mont  Blanc,  though  the  latter  exceeds  the  former  in 
height  by  500  feet.  The  fluctuations  of  the  barometer  are 
however  quite  sufficient  to  account  for  this  anomaly.  The 
lowering  of  the  boiling  point  is  about  1°  Fahr.  for  every 
590  feet  that  we  ascend;  and  from  the  temperature  at 
which  water  boils  we  may  approximately  infer  the  eleva- 
tion. It  is  said  that  to  make  good  tea  in  London,  boiling 
water  is  essential ;  if  this  be  so  it  is  evident  that  the  bever- 
age cannot  be  procured,  in  all  its  excellence,  at  the  higher 
stations  in  the  Alps. 

Let  us  now  make  an  experiment  to  illustrate  the  de- 
pendence of  the  boiling  point  on  external  pressure.  Here 
is  a  flask,  F  (fig.  35),  containing  water ;  here  is  another  and 
a  much  larger  one,  G,  from  which  I  have  had  the  air  re- 
moved by  an  air  pump.  The  two  flasks  are  connected  to- 
gether by  a  system  of  cocks,  which  enables  me  to  establish 
a  communication  between  them.  The  water  in  the  small 
flask  has  been  kept  boiling  for  some  time,  the  steam  gen- 
erated escaping  from  the  cock  y.  I  now  remove  the  spirit 


INFLUENCE   OF   PRESSURE   ON   BOILING   POINT.        133 

lamp  and  turn  this  cock  so  as  to  shut  o?it  the  air.  The 
water  ceases  to  boil,  and  pure  steam  now  fills  the  flask 
above  it.  Give  the  water  time  to  cool  a  little.  At  inter- 
vals you  see  a  bubble  of  steam  rising,  because  the  pressure 
of  the  vapour  above  is  gradually  becoming  less  through  its 
slow  condensation.  I  hasten  the  condensation  by  pouring 
cold  water  on  the  flask,  the  bubbles  are  more  copiously 
generated.  By  plunging  the  flask  bodily  into  cold  water 
we  might  cause  it  to  boil  violently.  The  water  is  now  at 

Fig.  35. 


rest  and  some  degrees  below  its  ordinary  boiling  point.    I 
turn  this  cock  c,  which  opens  a  way  for  the  escape  of  the 


134:  LECTURE   IV. 

vapour  into  the  exhausted  vessel  G  ;  the  moment  the  pres- 
sure is  diminished  ebullition  sets  in  in  F  ;  and  observe  how 
the  condensed  steam  showers  in  a  kind  of  rain  against  the 
sides  of  the  exhausted  vessel.  By  intentionally  promoting 
this  condensation,  and  thereby  preventing  the  vapour  in  the 
large  flask  from  reacting  upon  the  surface  of  the  water,  we 
can  keep  the  small  flask  bubbling  and  boiling  for  a  consid- 
erable length  of  time. 

By  high  heating,  the  elastic  force  of  steam  becomes 
enormous.  The  Marquis  of  Worcester  burst  cannon  with 
it,  and  our  calamitous  boiler  explosions  are  so  many  illus- 
trations of  its  power.  By  the  skill  of  man  this  mighty 
agent  has  been  controlled :  by  it  Denis  Papin  raised  a  pis- 
ton, which  was  pressed  down  again  by  the  atmosphere, 
when  the  steam  was  condensed ;  Savery  and  Newcomen 
turned  it  to  practical  account,  and  James  Watt  completed 
the  grand  application  of  the  moving  power  of  heat*  Push- 
ing the  piston  up  by  steam,  while  the  space  above  the  pis- 
ton is  in  communication  with  a  condenser  or  with  the  free 
air,  and  again  pushing  down  the  piston,  while  the  space 
below  it  is  in  communication  with  a  condenser  or  with 
the  air,  we  obtain  a  simple  to  and  fro  motion,  which,  by 
mechanical  arrangements,  may  be  made  to  take  any  form 
we  please. 

But  the  grand  principle  of  the  conservation  of  force  is 
illustrated  here  as  elsewhere.  For  every  stroke  of  work 
done  by  the  steam-engine,  for  every  pound  that  it  lifts,  and 
for  every  wheel  that  it  sets  in  motion,  an  equivalent  of  heat 
disappears.  A  ton  of  coal  furnishes  by  its  combustion  a 
certain  definite  amount  of  heat.  Let  this  quantity  of  coal 
be  applied  to  work  a  steam-engine ;  and  let  all  the  heat 
communicated  to  the  machine  and  the  condenser,  and  all 
the  heat  lost  by  radiation  and  by  contact  with  the  air  be 
collected ;  it  would  fall  short  of  the  amount  produced  by 
the  simple  combustion  of  the  ton  of  coal,  and  it  would  fall 


HEAT   AND   WORK   IN   THE   STEAM    ENGINE.  135 

short  of  it  by  an  amount  exactly  equivalent  to  the  quantity 
of  work  performed.  Suppose  that  work  to  consist  in  lift- 
ing a  weight  of  7,720  Ibs.  a  foot  high  ;  the  heat  produced 
by  the  coal  would  fall  short  of  its  maximum,  by  a  quantity 
just  sufficient  to  warm  a  pound  of  water  10°. 

But  my  object  in  these  lectures  is  to  deal  with  nature 
rather  than  art,  and  the  limits  of  our  time  compel  me 
to  pass  quickly  over  the  triumphs  of  man's  skill  in  the 
application  of  steam  to  the  purposes  of  life.  Those  who 
have  walked  through  the  workshops  of  Woolwich,  or 
through  any  of  orir  great  factories  where  machinery  is  ex- 
tensively employed,  will  have  been  sufficiently  impressed 
with  the  aid  which  this  great  power  renders  to  man.  And 
be  it  remembered,  every  wheel  which  revolves,  every  chis- 
el, and  plane,  and  saw,  and  punch,  which  forces  its  way 
through  solid  iron  as  if  it  were  so  much  cheese,  derives  its 
moving  energy  from  the  clashing  atoms  in  the  furnace. 
The  motion  of  these  atoms  is  communicated  to  the  boiler, 
thence  to  the  water,  whose  particles  are  shaken  asunder, 
and  fly  from  each  other  with  a  repellent  energy  commen- 
surate with  the  heat  communicated.  The  steam  is  simply 
the  apparatus  through  the  intermediation  of  which  the 
atomic  motion  is  converted  into  the  mechanical.  And  the 
motion  thus  generated  can  reproduce  its  parent.  Look  at 
the  planing  tools  ;  look  at  the  boring  instruments — streams 
of  water  gush  over  them  to  keep  them  cool.  Take  up  the 
curled  iron  shavings  which  the  planing  tool  has  pared  off: 
you  cannot  hold  them  in  your  hand  they  are  so  hot.  Here 
the  moving  force  is  restored  to  its  first  form ;  the  energy 
of  the  machine  has  been  consumed  in  reproducing  the 
power  from  which  that  energy  was  derived. 

I  must  now  direct  your  attention  to  a  natural  steam- 
engine  which  long  held  a  place  among  the  wonders  of  the 
world.  I  allude  to  the  Great  Geyser  of  Iceland.  The  sur- 
face of  Iceland  gradually  slopes  from  the  coast  towards  the 


136  LECTUEE   IV. 

centre,  where  the  general  level  is  about  2,000  feet  above 
the  sea.  On  this,  as  a  pedestal,  are  planted  the  Jokull  or 
icy  mountains,  which  extend  both  ways  in  a  north-easterly 
direction.  Along  this  chain  occur  the  active  volcanoes  of 
the  island,  and  the  thermal  springs  follow  the  same  general 
direction.  From  the  ridges  and  chasms  which  diverge  from 
the  mountains  enormous  masses  of  steam  issue  at  intervals 
hissing  and  roaring ;  and  when  the  escape  occurs  at  the 
mouth  of  a  cavern,  the  resonance  of  the  cave  often  raises 
the  sound  to  the  loudness  of  thunder.  Lower  down  in  the 
more  porous  strata  we  have  smoking  mud  pools,  where  a 
repulsive  blue-black  aluminous  paste  is  boiled,  rising  at 
times  in  hugh  bubbles,  which,  on  bursting,  scatter  their 
slimy  spray  to  a  height  of  fifteen  or  twenty  feet.  From 
the  bases  of  the  hills  upwards  extend  the  glaciers,  and 
above  these  are  the  snow-fields  which  crown  the  summits. 
From  the  arches  and  fissures  of  the  glaciers  vast  masses 
of  water  issue,  falling  at  times  in  cascades  over  walls  of 
ice,  and  spreading  for  miles  over  the  country  before  they 
find  definite  outlet.  Extensive  morasses  are  thus  formed, 
which  add  their  comfortless  monotony  to  the  dismal  scene 
already  before  the  traveller's  eye.  Intercepted  by  the 
cracks  and  fissures  of  the  land,  a  portion  of  this  water  finds 
its  way  to  the  heated  rocks  underneath  ;  and  here,  meeting 
with  the  volcanic  gases  which  traverse  these  underground 
regions,  both  travel  together,  to  issue,  at  the  first  conve 
nient  opportunity,  either  as  an  eruption  of  steam  or  a  boil- 
ing spring. 

The  most  famous  of  these  springs  is  the  Great  Geyser. 
It  consists  of  a  tube  74  feet  deep  and  10  feet  in  diameter. 
The  tube  is  surmounted  by  a  basin,  which  measures  from 
north  to  south  52  feet  across,  and  from  east  to  west  60 
feet.  The  interior  of  the  tube  and  basin  is  coated  with  a 
beautiful  smooth  siliceous  plaster,  so  hard  as  to  resist 
the  blows  of  a  hammer,  and  the  first  question  is,  how  was 


THE   GREAT  GEYSEK  OF  ICELAND.  137 

this  wonderful  tube  constructed — how  was  this  perfect 
plaster  laid  on  ?  Chemical  analysis  shows  that  the  water 
holds  silica  in  solution,  and  the  conjecture  might  therefore 
arise  that  the  water  had  deposited  the  silica  against  the 
sides  of  the  tube  and  basin.  But  this  is  not  the  case  :  the 
water  deposits  no  sediment ;  no  matter  how  long  it  may  be 
kept,  no  solid  substance  is  separated  from  it.  It  may  be 
bottled  up  and  preserved  for  years  as  clear  as  crystal,  with- 
out showing  the  slightest  tendency  to  form  a  precipitate. 
To  answer  the  question  in  this  way  would  moreover  assume 
that  the  shaft  was  formed  by  some  foreign  agency,  and 
that  the  water  merely  lined  it.  The  geyser  basin,  however, 
rests  upon  the  summit  of  a  mound  about  40  feet  high,  and 
it  is  evident,  from  mere  inspection,  that  the  mound  has 
been  deposited  by  the  geyser.  But  in  building  up  this 
mound  the  spring  must  have  formed  the  tube  which  per- 
forates the  mound,  and  hence  the  conclusion  that  the  gey- 
ser is  the  architect  of  its  own  tube. 

If  we  place  a  quantity  of  the  geyser  water  in  an  evapor- 
ating basin  the  following  takes  place  :  In  the  centre  of  the 
basin  the  liquid  deposits  nothing,  but  at  the  sides,  where 
it  is  drawn  up  by  capillary  attraction,  and  thus  subjected 
to  speedy  evaporation,  we  find  silica  deposited.  Round 
the  edge  a  ring  of  silica  is  laid  on,  and  not  until  the  evapo- 
ration has  continued  a  considerable  time  do  we  find  the 
slightest  turbidity  in  the  middle  of  the  water.  This  exper- 
iment is  the  microscopic  representant  of  what  occurs  in 
Iceland.  Imagine  the  case  of  a  simple  thermal  siliceous 
spring,  whose  waters  trickle  down  a  gentle  incline ;  the 
water  thus  exposed  evaporates  speedily,  and  silica  is  de- 
posited. This  deposit  gradually  elevates  the  side  over 
which  the  water  passes  until  finally  the  latter  has  to  take 
another  course.  The  same  takes  place  here,  the  ground  is 
elevated  as  before  and  the  spring  has  to  move  forward. 
Thus  it  is  compelled  to  travel  round  and  round,  discharg- 


138  LECTURE   IV. 

ing  its  silica  and  deepening  the  shaft  in  which  it  dwells, 
until  finally,  in  the  course  of  ages,  the  simple  spring  has 
produced  that  wonderful  apparatus  which  has  so  long  puz- 
zled and  astonished  both  the  traveller  and  the  philosopher. 

Previous  to  an  eruption,  both  the  tube  and  basin  are 
filled  with  hot  water ;  detonations  which  shake  the  ground, 
are  heard  at  intervals,  and  each  is  succeeded  by  a  violent 
agitation  of  the  water  in  the  basin.  The  water  in  the  pipe 
is  lifted  up  so  as  to  form  an  eminence  in  the  middle  of  the 
basin,  and  an  overflow  is  the  consequence.  These  detona- 
tions are  evidently  due  to  the  production  of  steam  in  the 
ducts  which  feed  the  geyser  tube,  which  steam  escaping 
into  the  cooler  water  of  the  tube  is  there  suddenly  con- 
densed, and  produces  the  explosions.  Professor  Bunsen 
succeeded  in  determining  the  temperature  of  the  geyser 
tube,  from  top  to  bottom,  a  few  minutes  before  a  great 
eruption ;  and  these  observations  revealed  the  extraordi- 
nary fact,  that  at  no  part  of  the  tube  did  the  water  reach 
its  boiling  point.  In  the  annexed  sketch  (fig.  36)  I  have 
given,  on  one  side,  the  temperatures  actually  observed,  and 
on  the  other  side  the  temperatures  at  which  water  would 
boil,  taking  into  account  both  the  pressure  of  the  atmos- 
phere and  the  pressure  of  the  superincumbent  column  of 
water.  The  nearest  approach  to  the  boiling  point  is  at  A, 
a  height  of  30  feet  from  the  bottom ;  but  even  here  the 
water  is  2°  Centigrade,  or  more  than  3-i°  Fahr.  below  the 
temperature  at  which  it  could  boil.  How  then  is  it  pos- 
sible that  an  eruption  could  occur  under  such  circum- 
stances ? 

Fix  your  attention  upon  the  water  at  the  point  A  ; 
where  the  temperature  is  within  2°  C.  of  the  boiling  point. 
Call  to  mind  the  lifting  of  the  column  when  the  detona- 
tions are  heard.  Let  us  suppose  that  by  the  entrance  of 
steam  from  the  ducts  near  the  bottom  of  the  tube,  the 
geyser  column  is  elevated  6  feet,  a  height  quite  within  the 


THEOKY  OF  THE  GEYSEB. 


139 


limits  of  actual  observation ;  the  water  at  A  is  thereby 
transferred  to  B.  Its  boiling  point  at  A  is  123*8°,  and  its 
actual  temperature  121'8°  ;  but  at  B  its  boiling  point  is 
only  120*8°,  hence,  when  transferred  from  A  to  B  the  heat 
which  it  possesses  is  in  excess  of  that  necessary  to  make  it 
boil.  This  excess  of  heat  is  instantly  applied  to  the  gen- 
eration of  steam :  the  column  is  thus  lifted  higher,  and  the 


85.5- 


OPSERVED 
TEMPERATURES 


12* 


126°. 


-10  FEET- 


BOILING 
TEMPERATURES 


120.8 


-130" 


I35a 


water  below  is  further  relieved.    More  steam  is  generated ; 
from  the  middle  downwards  the  mass  suddenly  bursts  into 


140 


LECTURE  IV. 


Fig  37. 


GEYSEK   OPERATIONS   IN   ICELAND.  141 

ebullition,  the  water  above,  mixed  with  steam  clouds,  is 
projected  into  the  atmosphere,  and  we  have  the  geyser 
eruption  in  all  its  grandeur. 

By  its  contact  with  the  air  the  water  is  cooled,  falls 
back  into  the  basin,  partially  refills  the  tube,  in  which  it 
gradually  rises,  and  finally  fills  the  basin  as  before.  Deto- 
nations are  heard  at  intervals,  and  risings  of  the  water  in 
the  basin.  These  are  so  many  futile  attempts  at  an  erup- 
tion, for  not  until  the  water  in  the  tube  comes  sufficiently 
near  its  boiling  temperature,  to  make  the  lifting  of  the  col- 
umn effective,  can  we  have  a  true  eruption. 

To  Bunsen  we  owe  this  beautiful  theory,  and  now  let 
us  try  to  justify  it  by  experiment.  Here  is  a  tube  of  gal- 
vanized iron,  6  fee't  long,  A  B  (fig.  37),  and  surmounted  by 
this  basin  c  D.  It  is  heated  by  a  fire 

underneath ;  and  to  imitate  as  far  as i 

possible  the  condition  of  the  geyser, 
I  have  encircled  the  tube  by  a  second 
fire  F,  at  a  height  of  2  feet  from  the 
bottom.  Doubtless  the  high  tem- 
perature of  the  water  at  the  corres- 
ponding part  of  the  geyser  tube  is 
due  to  a  local  action  of  the  heated 
rocks.  I  fill  the  tube  with  water, 
which  gradually  becomes  heated  ;  and 
regularly,  every  five  minutes,  the  wa- 
ter is  ejected .  from  the  tube  into  the 
atmosphere. 

But  there  is  another  famous  spring 
in  Iceland,  called  the  Strokkur,  which 
is  usually  forced  to  explode  by  stop- 
ping its  mouth  with  clods.  We  can 
imitate  the  action  of  this  spring  by 
stopping  the  mouth  of  our  tube  A  B  with  a  cork.  I  do 
so :  and  now  the  heating  progresses.  The  steam  below 


142  LECTURE  IV. 

will  finally  attain  sufficient  tension  to  eject  the  cork,  and 
the  water,  suddenly  relieved  from  the  pressure,  will 
burst  forth  in  the  atmosphere.  There  it  goes !  The 
ceiling  of  this  room  is  nearly  30  feet  from  the  floor,  but 
the  eruption  has  reached  the  ceiling,  from  which  the 
water  now  drips  plentifully.  In  fig.  38,  I  have  given  a 
section  of  the  Strokkur. 

By  stopping  the  tube  with  corks,  through  which  tubes 
of  various  lengths  and  widths  pass,  the  action  of  many  of 
the  other  eruptive  springs  may  be  accurately  imitated. 
Here,  for  example,  I  have  an  intermittent  action ;  dis- 
charges of  water  and  impetuous  steam  gushes  follow  each 
other  in  quick  succession,  the  water  being  squirted  in  jets 
15  or  20  feet  high.  Thus,  it  is  proved  experimentally, 
that  the  geyser  tube  itself  is  the  sufficient  cause  of  the 
eruptions,  and  we  are  relieved  from  the  necessity  of  ima- 
gining underground  caverns  filled  with  water  and  steam, 
which  were  formerly  regarded  as  necessary  to  the  produc- 
tion of  these  wonderful  phenomena. 

A  moment's  reflection  will  suggest  to  us  that  there 
must  be  a  limit  to  the  operations  of  the  geyser.  When 
the  tube  has  reached  such  an  altitude  that  the  water  in  the 
depths  below,  owing  to  the  increased  pressure,  cannot  at- 
tain its  boiling  point,  the  eruptions  of  necessity  cease.  The 
spring,  however,  continues  to  deposit  its  silica,  and  often 
forms  a  Laug  or  cistern.  Some  of  those  in  Iceland  are  40 
feet  deep.  Their  beauty,  according  to  Bunsen,  is  inde- 
scribable ;  over  the  surface  curls  a  light  vapour,  the  water 
is  of  the  purest  azure,  and  tints  with  its  lovely  hue  the  fan- 
tastic incrustations  on  the  cistern  walls ;  while,  at  the  bot- 
tom, is  often  seen  the  mouth  of  the  once  mighty  geyser. 
There  are  in  Iceland  vast,  but  now  extinct,  geyser  opera- 
tions. Mounds  are  observed  whose  shafts  are  filled  with 
rubbish,  the  water  having  forced  a  passage  underneath  and 
retired  to  other  scenes  of  action.  We  have  in  fact  the 


GEYSER   OPERATIONS   IN   ICELAND.  143 

geyser  in  its  youth,  manhood,  old  age,  and  death,  here  pre- 
sented to  us.  In  its  youth,  as  a  simple  thermal  spring  ;  in 
its  manhood,  as  the  eruptive  column ;  in  its  old  age,  as  the 
tranquil  Lang  •  while  its  death  is  recorded  by  the  ruined 
shaft  and  mound  which  testify  the  fact  of  its  once  active 
existence. 


APPENDIX  TO  LECTURE  IV. 


ABSTRACT  OF  A  LECTURE  ON  THE  VIBRATION  AND  TONES  PRO- 
DUCED BY  THE  CONTACT  OF  BODIES  OF  DIFFERENT  TEMPER- 
ATURES. 

[Given  at  tlie  Royal  Institution  on  Friday,  January  27, 1854.] 

IN  the  year  1805,  M.  Schwartz,  an  inspector  of  one  of  the 
smelting  works  in  Saxony,  placed  a  cup-shaped  mass  of  hot  silver 
upon  a  cold  anvil,  and  was  surprised  to  find  that  musical  tones 
proceeded  from  the  mass.  In  the  autumn  of  the  same  year,  Pro- 
fessor Gilbert  of  Berlin  visited  the  smelting  works  and  repeated 
the  experiment.  He  observed,  that  the  sounds  were  accompanied 
by  a  quivering  of  the  hot  silver,  and  that  when  the  vibrations 
ceased,  the  sound  ceased  also.  Professor  Gilbert  merely  stated  the 
facts,  and  made  no  -attempt  to  explain  them. 

In  the  year  1829,  Mr.  Arthur  Trevelyan,  being  engaged  in 
spreading  pitch  with  a  hot  plastering  iron,  and  once  observing 
that  the  iron  was  too  hot  for  his  purpose,  he  laid  it  slantingly 
against  a  block  of  lead  which  chanced  to  be  at  hand ;  a  shrill 
note,  which  he  compared  to  that  of  the  chanter  of  the  small 
Northumberland  pipes,  proceeded  from  the  mass,  and,  on  nearer 
inspection,  he  observed  that  the  heated  iron  was  in  a  state  of  vi- 
bration. He  was  induced  by  Dr.  Reid  of  Edinburgh  to  pursue 
the  subject,  and  the  results  of  his  numerous  experiments  were 
subsequently  printed  in  the  Transactions  of  the  Royal  Society  of 
Edinburgh. 

On  April  1,  1831,  these  singular  sounds  and  vibrations  formed 
the  subject  of  a  Friday  evening  discourse  by  Professor  Faraday  at 
the  Royal  Institution.  Professor  Faraday  expanded  and  further 
established  the  explanation  of  the  sounds  given  by  Mr.  Trevelyan 


GENERAL   LAWS   OF   PROFESSOR   FORBES.  145 

and  Sir  John  Leslie.  He  referred  them  to  the  tapping  of  the  hot 
mass  against  the  cold  one  underneath  it,  the  taps  being  in  many 
cases  sufficiently  quick  to  produce  a  high  musical  note.  The  al- 
ternate expansion  and  contraction  of  the  cold  mass  at  the  points 
where  the  hot  rocker  descends  upon  it,  he  regarded  as  the  sus- 
taining power  of  the  vibrations.  The  superiority  of  lead  he  as- 
cribed to  its  great  expansibility,  combined  with  its  feeble  power 
of  conduction,  which  latter  prevented  the  heat  from  being  quick- 
ly diffused  through  the  mass. 

Professor  J.  D.  Forbes  of  Edinburgh  was  present  at  this  lec- 
ture, and  not  feeling  satisfied  with  the  explanation,  undertook  the 
farther  examination  of  the  subject ;  his  results  are  described  in 
a  highly  ingenious  paper  communicated  to  the  Royal  Society  of 
Edinburgh  in  1833.  He  rejects  the  explanation  supported  by 
Professor  Faraday,  and  refers  the  vibrations  to  '  a  new  species  of 
mechanical  agency  in  heat ' — a  repulsion  exercised  by  the  heat 
itself  on  passing  from  a  good  conductor  to  a  bad  one.  This  con- 
clusion is  based  upon  a  number  of  general  laws  established  by 
Professor  Forbes.  If  these  laws  be  correct,  then  indeed  a  great 
step  has  been  taken  towards  a  knowledge  of  the  intimate  nature 
of  heat  itself,  and  this  consideration  was  the  lecturer's  principal 
stimulus  in  resuming  the  examination  of  the  subject. 

He  had  already  made  some  experiments,  ignorant  that  the  sub- 
ject had  been  farther  treated  by  Seebcck,  until  informed  of  the 
fact  by  Professor  Magnus  of  Berlin.  On  reading  Seebeck's  inter- 
esting paper,  he  found  that  many  of  the  results  which  it  was  his 
intention  to  seek  had  been  already  obtained.  The  portion  of  the 
subject  which  remained  untouched  was,  however,  of  sufficient  in- 
terest to  induce  him  to  prosecute  his  original  intention. 

The  general  laws  of  Professor  Forbes  were  submitted  in  succes- 
sion to  an  experimental  examination.  The  first  of  these  laws 
affirms  that '  the  vibrations  never  take  place  between  substances  of  the 
same  nature?  This  the  lecturer  found  to  be  generally  the  case  when 
the  hot  rocker  rested  upon  a  llock,  or  on  the  edge  of  a  thick  plate 
of  the  same  metal ;  but  the  case  was  quite  altered  when  a  thin 
plate  of  metal  was  used.  Thus  a  copper  rocker  laid  upon  the 
-edge  of  a  penny-piece  did  not  vibrate  permanently ;  but  when  the 
coin  was  beaten  out  by  a  hammer,  so  as  to  present  a  thin  sharp 
edge,  constant  vibrations  were  obtained.  A  silver  rocker  resting 
7 


146  APPENDIX  TO   LECTURE  IV. 

resting  on  the  edge  of  a  half-crown  refused  to  vibrate  permanently ; 
but  on  the  edge  of  a  sixpence  continuous  vibrations  were  obtained. 
An  iron  rocker  on  the  edge  of  a  dinner  knife  gave  continuous  vi- 
brations. A  flat  brass  rocker  placed  upon  the  points  of  two  com- 
mon brass  pins,  and  having  its  handle  suitably  supported,  gave 
distinct  vibrations.  In  these  experiments  the  plates  and  pins 
w ere  fixed  in  a  vice,  and  it  was  found  that  the  thinner  the  plate, 
within -its  limits  of  rigidity,  the  more  certain  and  striking  was  the 
effect.  Vibrations  were  thus  obtained  with  iron  on  iron,  copper 
on  copper,  brass  on  brass,  zinc  on  zinc,  silver  on  silver,  tin  on 
tin.  The  list  might  be  extended,  but  the  cases  cited  are  sufficient 
to  show  that  the  proposition  above  cited  cannot  be  regarded  as 
expressing  a  *  general  law.' 

The  second  general  law  enunciated  by  Professor  Forbes  is,  that 
'  l)oth  substances  must  le  metallic.'1  This  is  the  law  which  first  at- 
tracted the  lecturer's  attention.  During  the  progress  of  a  kindred 
enquiry,  he  had  discovered  that  certain  non-metallic  bodies  are 
endowed  with  powers  of  conduction  far  higher  than  has  been 
hitherto  supposed,  and  the  thought  occurred  to  him  that  such 
bodies  might,  by  suitable  treatment,  be  made  to  supply  the  place 
of  metals  in  the  production  of  vibrations.  This  anticipation  was 
realized.  Rocks  of  silver,  copper,  and  brass,  placed  upon  the 
natural  edge  of  a  prism  of  rock  crystal,  gave  distinct  tones ;  on 
the  clean  edge  of  a  cube  of  fluor  spar,  the  tones  were  still  more 
musical ;  on  a  mass  of  rock-salt  the  vibrations  were  very  forcible. 
There  is  scarcely  a  substance,  metallic  or  non-metallic,  on  which 
vibrations  can  be  obtained  with  greater  ease  and  certainty  than 
on  rock-salt.  In  most  cases  a  high  temperature  is  necessary  to 
the  production  of  the  tones,  but  in  the  case  of  rock-salt  the  tem- 
perature need  not  exceed  that  of  the  blood.  A  new  and  singular 
property  is  thus  found  to  belong  to  this  already  remarkable  sub- 
stance. It  is  needless  to  enter  into  a  full  statement  regarding  the 
various  minerals  submitted  to  experiment.  Upwards  of  twenty 
non-metallic  substances  had  been  examined  by  the  lecturer,  and 
distinct  vibrations  obtained  with  every  one  of  them. 

The  number  of  exceptions  here  exhibited'  far  exceeds  that  of 
the  substances  which  are  mentioned  in  the  paper  of  Professor 
Forbes,  and  are,  it  was  imagined,  sufficient  to  show  that  the  sec- 
ond general  law  is  untenable. 


LAWS   TESTED   EXPERIMENTALLY. 

The  third  general  law  states,  that  *  the  mirations  tale  place 
with  an  intensity  proportional  (within  certain  limits)  to  the  differ- 
ence of  the  conducting  powers  of  tlie  metals  for  heat,  tlie  metal  having 
the  least  conducting  power,  being  necessarily  the  coldest.''  The  evi- 
dence adduced  against  the  first  law  appears  to  destroy  this  one 
also  ;  for  if  the  intensity  of  the  vibrations  be  proportional  to  the 
difference  of  the  conducting  powers,  then,  where  there  is  no  such 
difference,  there  ought  to  be  no  vibrations.  But  it  has  been 
proved  in  half  a  dozen  cases,  that  vibrations  occur  between  differ- 
ent pieces  of  the  same  metal..  The  condition  stated  by  Professor 
Forbes  was,  however,  reversed.  Silver  stands  at  the  head  of  con- 
ductors ;  a  strip  of  the  metal  was  fixed  in  a  vice,  and  hot  rockers 
of  brass,  copper,  and  iron,  were  successively  laid  upon  its  edge : 
distinct  vibrations  were  obtained  with  all  of  them.  Vibrations 
were  also  obtained  with  a  brass  rocker  which  rested  on  the  edge 
of  a  half-sovereign.  These  and  other  experiments  show  that  it  is 
not  necessary  that  the  worst  conductor  should  be  the  cold  metal, 
as  affirmed  in  the  third  general  law  above  quoted.  Among  the 
metals,  antimony  and  bismuth  were  found  perfectly  inert  by 
Professor  Forbes ;  the  lecturer  however  had  obtained  musical 
tones  from  both  of  these  substances. 

The  superiority  of  lead  as  a  cold  block,  Professor  Faraday,  as 
already  stated,  referred  to  its  high  expansibility,  combined  with 
its  deficient  conducting  power.  Against  this  notion,  which  he 
considers  to  be  '  an  obvious  oversight,'  Professor  Forbes  contends 
in  an  ingenious  and  apparently  unanswerable  manner.  The  vi- 
brations, he  urges,  depend  upon  the  difference  of  temperature 
existing  between  the  rocker  and  the  block  ;  if  the  latter  be  a  bad 
conductor  and  retain  the  heat  at  its  surface,  the  tendency  is  to 
bring  both  the  surfaces  in  contact  to  the  same  temperature,  and 
thus  to  stop  the  vibration  instead  of  exalting  it.  Farther :  the 
greater  the  quantity  of  heat  transmitted  from  the  rocker  to  the 
block  during  contact,  the  greater  must  be  the  expansion ;  and 
hence,  if  the  vibrations  be  due  to  this  cause,  the  effect  must  be  a 
maximum  when  the  block  is  the  best  conductor  possible.  But 
Professor  Forbes,  in  this  argument,  seems  to  have  used  the  term 
expansion  in  two  different  senses.  The  expansion  which  produces 
the  vibration  is  the  sudden  upheaval  of  the  point  where  the 
hot  rocker  comes  in  contact  with  the  cold  mass  underneath  :  but 


14:8  APPENDIX   TO   LECTURE   IV. 

the  expansion  due  to  good  conduction  would  be  an  expansion  of 
the  general  mass.  Imagine  the  conductive  power  of  the  block  to 
be  infinite — that  is  to  say,  that  the  heat  imparted  by  the  rocker  is 
instantly  diffused  equally  throughout  the  block;  then,  though 
the  general  expansion  might  be  very  great,  the  local  expansion  at 
the  point  of  contact  would  be  wanting,  and  no  vibrations  would 
be  possible.  The  inevitable  consequence  of  good  conduction  is, 
to  cause  a  sudden  abstraction  of  the  heat  from  the  point  of  con- 
tact of  the  rocker  with  the  substance  underneath,  and  this  the 
lecturer  conceived  to  be  the  precise  reason  why  Professor  Forbes 
had  failed  to  obtain  vibrations  when  the  cold  metal  was  a  good 
conductor.  He  made  use  of  llocJcs,  and  the  abstraction  of  heat 
from  the  place  of  contact  by  the  circumjacent  mass  of  metal,  was 
so  sudden  as  to  extinguish  the  local  elevation  on  which  the  vibra- 
tions depend.  In  the  experiments  described  by  the  lecturer,  this 
abstraction  was  to  a  great  extent  avoided,  by  reducing  the  metal- 
lic masses  to  thin  laminae ;  and  thus  the  very  experiments  adduced 
by  Professor  Forbes  against  the  theory  supported  by  Professor 
Faraday,  appear,  when  duly  considered,  to  be  converted  into 
strong  corroborative  proofs  of  the  correctness  of  the  views  of  the 
philosopher  last  mentioned. 


EXTRACT  FBOM  A  PAPER  ON  SOME  PHYSICAL  PEOPEETIES  OF 

ICE* 

In  a  very  interesting  paper  communicated  to  the  British  Asso- 
ciation during  its  last  meeting,  Mr.  James  Thomson  has  explained 
the  freezing  together  of  two  pieces  of  ice.at  32°  Fahr.,  in  the  fol- 
lowing manner : — '  The  two  pieces  of  ice,  on  being  pressed  to- 
gether at  their  point  of  contact,  will  at  that  place,  in  virtue  of  the 
pressure,  be  in  part  liquefied  and  reduced  in  temperature,  and  the 
cold  evolved  in  their  liquefaction  will  cause  some  of  the  liquid 
film  intervening  between  the  two  masses  to  freeze.' 

I  am  far  from  denying  the  operation  under  proper  circum- 
stances of  the  vera  causa  to  which  Mr.  Thomson  refers,  but  I  do 

*  Phil.  Trans.,  1858,  p.  225. 


LAMINATION   OF   ICE   BY  PKESSUEE. 


149 


not  think  it  explains  the  facts.  For  freezing  takes  place  without 
the  intervention  of  any  pressure  by  which  Mr.  Thomson's  effect 
could  sensibly  come  into  play. 

It  is  not  necessary  to  squeeze  the  pieces  of  ice  together ;  one 
bit  may  be  simply  laid  upon  the  other,  and  they  will  still  freeze. 
Other  substances  besides  ice  are  also  capable  of  being  frozen  to 
the  ice.  If  a  towel  be  folded  round  a  piece  of  ice  at  32°  the  towel 
and  ice  will  freeze  together.  Flannel  is  still  better ;  a  piece  of 
flannel  wrapped  round  a  piece  of  ice,  freezes  to  it  sometimes  so 
firmly  that  a  strong  tearing  force  is  necessary  to  separate  both. 
Cotton,  wool,  and  hair  may  also  be  frozen  to  ice,  without  the  in- 
tervention of  any  pressure  which  would  render  Mr.  Thomson's 
cause  sensibly  active. 

But  there  is  a  class  of  effects  to  the  explanation  of  which  the 
lowering  of  the  freezing  point  of  water,  by  pressure,  may,  I  think, 
be  properly  applied.  The  following  statement  is  true  of  fifty  ex- 
periments, or  more,  made  with  ice  from  various  quarters.  A  cyl- 
inder of  ice,  two  inches  high  and  an  inch  in  diameter,  was  placed 
between  two  slabs  of  box-wood,  and  submitted  to  a  gradually 
increasing  pressure.  Looked  at  perpendicularly  to  the  axis, 
cloudy  lines  were  seen  drawing  themselves  across  the  cylinder ; 
and  when  the  latter  was  looked  at  obliquely,  these  lines  were 
found  to  be  sections  of  dim  hazy  surfaces  which  traversed  the 
cylinder,  and  gave  it  an  appearance  closely  resembling  that  of  a 
crystal  of  gypsum  whose  planes  of  cleavage  had  been  forced  out 
of  optical  contact  by  some  external  force. 

Fig.  39  represents  the  cylinder  looked  at  perpendicularly  to 
its  axis,  and  fig.  40  the  same  cylinder  when  looked  at  obliquely. 

Fig.  39.  Fig.  40. 


150 


APPENDIX   TO   LECTURE   IV. 


To  ascertain  whether  the  rupture  of  optical  contact  which 
these  experiments  disclosed  was  due  to  the  intrusion  of  air  be- 
tween two  separated  surfaces  of  ice,  a  cylinder  of  ice,  two  inches 
long  and  one  inch  wide,  was  placed  in  a  copper  vessel  containing 
ice-cold  water.  The  ice  cylinder  projected  half  an  inch  above  the 
surface  of  the  water.  Placing  the  copper  vessel  on  a  slab  of 
wood,  and  a  second  slab  of  wood  upon  the  cylinder  of  ice,  the 
whole  was  subjected  to  pressure.  "When  the  hazy  surfaces  were 
well  developed  in  the  portion  of  ice  above  the  water,  the  cylinder 
was  removed  and  examined.  The  planes  of  rupture  extended 
throughout  the  entire  length  of  the  cylinder,  just  the  same  as  it 
had  been  squeezed  in  free  air. 

Still  the  removal  of  the  cylinder  from  its  vessel  might  be  at- 
tended with  the  intrusion  of  air  into  the  fissures.  I  therefore 
placed  a  cylinder  of  ice,  two  inches  long  and  one  inch  wide,  in  a 
stout  vessel  of  glass,  which  was  filled  with  ice-cold  water.  Squeez- 
ing the  whole,  as  in  the  last  experiment,  the  surfaces  of  discontin- 
uity were  seen  under  the  liquid  quite  as  distinctly  as  in  air. 

The  surfaces  are  due  to  compression,  and  not  to  any  tearing 
asunder  of  the  mass  by  tension,  and  they  are  best  developed 
where  the  pressure,  within  the  limits  of  fracture,  is  a  maximum. 
A  cylindrical  piece  of  ice,  one  of  whose  ends  was  not  parallel  to 
the  other,  was  placed  between  slabs  of  wood  and  subjected  to 
pressure.  Fig.  41  shows  the  disposition  of  the  experiment.  The 

Fig.  42. 


effect  upon  the  ice-cylinder  was  that  shown  in  fig.  42,  the  sur- 
faces being  developed  along  that  side  which  had  suffered  the 
pressure. 


LIQUEFACTION   OF   ICE   BY   PEESSUEE.  151 

Sometimes  the  surfaces  commence  at  the  centre  of  the  cylin- 
der. A  dim  small  spot  is  first  observed,  which,  as  the  pressure 
continues,  expands  until  it  sometimes  embraces  the  entire  trans- 
verse section  of  the  cylinder. 

On  examining  these  surfaces  with  a  pocket  lens,  they  appeared 
to  me  to  be  composed  of  very  minute  water-parcels,  like  what  is 
produced  upon  a  smooth  cold  surface  by  the  act  of  breathing. 
Were  they  either  vacuous  plates,  or  plates  filled  with  air,  their 
aspect  would,  on  optical  grounds,  be  far  more  vivid  than  it  really 
was. 

A  concave  mirror  was  so  disposed,  that  the  diffused  light  of 
day  was  thrown  full  upon  the  cylinder  while  under  pressure. 
Observing  the  expanding  surfaces  through  a  lens,  they  appeared 
in  a  state  of  intense  commotion ;  this  was  probably  due  to  the 
molecular  tensions  of  the  little  water-parcels.  This  motion  fol- 
lowed closely  on  the  edge  of  the  surface  as  it  advanced  through 
the  solid  ice.  Once  or  twice  I  observed  the  hazy  surfaces  pio- 
neered through  the  mass  by  dim  offshoots  apparently  liquid. 
They  constituted  a  kind  of  negative  crystallization,  having  the 
exact  form  of  the  crystalline  spines  and  spurs  produced  by  the 
congelation  of  water  upon  a  surface  of  glass.  I  have  no  doubt, 
then,  that  these  surfaces  are  produced  by  the  liquefaction  of  the 
solid  in  planes  perpendicular  to  the  direction  of  pressure. 

The  surfaces  are  developed  with  great  facility  when  they  cor- 
respond to  the  surfaces  of  freezing.  By  care  I  succeeded  in  some 
cases  in  producing  similar  effects  in  surfaces  at  right  angles  to  the 
planes  of  freezing,  but  this  was  difficult  and  uncertain.  Wherever 
the  liquid  disks  before  described  were  observed,  the  surfaces  were 
always  easily  developed  in  the  planes  of  the  disks. 


LECTURE    V. 

[February  20,  1862.] 

APPLICATION  OP  THE  DYNAMICAL  THEORY  TO  THE  PHENOMENA  OF  SPECIFIC 
AND  LATENT  HEAT — DEFINITION  OF  ENERGY  :  POTENTIAL  AND  DYNAMIC 
ENERGY — ENERGY  OF  MOLECULAR  FORCES — EXPERIMENTAL  ILLUSTRA- 
TIONS OF  SPECIFIC  AND  LATENT  HEAT — MECHANICAL  VALUES  OF  THE 
ACTS  OF  COMBINATION,  CONDENSATION,  AND  CONGELATION  IN  THE  CASE 
OF  WATER — SOLID  CARBONIC  ACID — THE  SPHEROIDAL  STATE  OF  LIQUIDS — 
FLOATING  OF  SPHEROID  ON  ITS  OWN  VAPOUR — FREEZING  OF  WATER  AND 

MERCURY  IN  A  RED-HOT' CRUCIBLE. 

"VTTHENEVER  a  difficult  expedition  is  undertaken  in 
VV  the  Alps,  the  experienced  mountaineer  commences 
the  day  at  a  slow  pace,  so  that  when  the  real  hour  of  trial 
arrives,  he  may  find  himself  hardened  instead  of  exhausted 
by  his  previous  work.  We,  to-day,  are  about  to  enter  on 
a  difficult  ascent,  and  I  propose  that  we  commence  it  in  the 
same  spirit ;  not  with  a  flush  of  enthusiasm  which  the 
necessity  of  labour  extinguishes,  but  with  patient  and 
determined  hearts  which  will  not  recoil  should  a  difficulty 
arise. 

I  have  here  a  lead  weight  attached  to  a  string  which 
passes  over  a  pulley  at  the  top  of  the  room.  We  know 
that  the  earth  and  the  weight  are  mutually  attractive  ;  the 
weight  now  rests  upon  the  earth  and  exerts  a  certain  press- 
ure upon  its  surface.  The  earth  and  the  weight  here 
touch  each  other  •  their  mutual  attractions  are  as  far  as 
possible  satisfied,  and  motion  by  their  mutual  approach  is 
no  longer  possible.  As  far  as  the  attraction  of  gravity  is 


POTENTIAL  AND  DYNAMIC  ENERGY.         153 

concerned,  the  possibility  of  producing  motion  ceases  as 
soon  as  the  two  attracting  bodies  are  actually  in  contact. 

I  draw  up  this  weight.  It  is  now  suspended  at  a  height 
of  sixteen  feet  above  the  floor ;  it  is  just  as  motionless  as 
when  it  rested  on  the  floor ;  but  by  introducing  a  space 
between  the  floor  and  it,  I  entirely  change  the  condition 
of  the  weight.  By  raising  it  I  have  conferred  upon  it  a 
motion-producing  power.  There  is  now  an  action  possible 
to  it,  which  was  not  possible  when  it  rested  upon  the  earth ; 
it  can  fall,  and  in.  its  descent  can  turn  a  machine  or  per- 
form other  work.  It  has  no  energy  as  it  hangs  there  dead 
and  motionless  ;  but  energy  is  possible  to  it,  and  we  might 
fairly  use  the  term  possible  energy,  to  express  this  power 
of  motion  which  the  weight  possesses,  but  which  has  not 
yet  been  exercised  by  falling  ;  or  we  might  call  it  '  poten- 
tial energy,'  as  some  eminent  men  have  already  done.  This 
potential  energy  is  derived,  in  the  case  before  us,  from  the 
pull  of  gravity,  which  pull,  however,  has  not  yet  eventuated 
in  motion.  But  I  now  let  the  string  go  ;  the  weight  falls, 
and  reaches  the  earth's  surface  with  a  velocity  of  thirty-two 
feet  a  second.  At  every  moment  of  descent  it  was  pulled 
down  by  gravity,  and  its  final  moving  force  is  the  summa- 
tion of  the  pulls.  While  in  the  act  of  falling,  the  energy 
of  the  weight  is  active.  It  may  be  called  actual  energy,  in 
antithesis  to  possible  ;  or  it  may  be  called  dynamic  energy, 
in  antithesis  to  potential,  or  we  might  call  the  energy  with 
which  the  weight  descends  moving  force.  Do  not  be  inat- 
tentive to  these  points  ;  we  must  be  able  promptly  to  dis- 
tinguish between  energy  in  store  and  energy  in  action. 
Once  for  all  then,  let  us  take  the  terms  of  Mr.  Rankine, 
and  call  the  energy  in  store  '  potential,'  and  the  energy  in 
action  4  actual.'  *  If,  after  this,  I  should  use  the  terms 

*  Helmhqltz,  in  his  admirable  memoir  on  '  Die  Erhaltung  der  Kraft,' 
(1847),  divided  all  energy  into  Tension  and  vis  viva.  (Spannkriifte  und 
Lebendige  Krafte.) 

7* 


154  LECTURE   V 

1  possible  energy,'  or  c  dynamic  energy,'  or  '  moving  force,' 
you  will  have  no  difficulty  in  affixing  the  exact  idea  to  these 
terms.  And  remember  exactness  is  here  essential.  We 
must  not  now  tolerate  vagueness  in  our  conceptions. 

Our  weight  started  from  a  height  of  sixteen  feet ;  let 
us  fix  our  attention  upon  it  after  it  has  accomplished  the 
first  foot  of  its  fall.  The  total  pull,  if  I  may  use  the  term, 
to  be  expended  on  it  has  been  then  diminished  by  the 
amount  expended  in  its  passing  through  the  first  foot.  At 
the  height  of  fifteen  feet  it  has  one  foot  less  of  potential 
energy  than  it  possessed  at  the  height  of  sixteen  feet,  but 
at  the  height  of  fifteen  feet  it  has  got  an  equivalent  amount 
of  dynamic  or  actual  energy  which,  if  reversed  in  direction, 
would  raise  it  again  to  its  primitive  height.  Hence  as  po- 
tential energy  disappears,  dynamic  energy  comes  into  play. 
Throughout  the  universe  the  sum  of  these  two  energies  is 
constant. 

It  is  as  yet  too  early  to  refer  to  organic  processes,  but 
could  we  observe  the  molecular  condition  of  my  arm  as  I 
drew  up  that  weight,  it  would  be  seen  that  in  accomplish- 
ing this  mechanical  act,  an  equivalent  amount  of  some 
other  form  of  motion  was  consumed.  If  the  weight  were 
raised  by  common  heat,  a  portion  of  heat  would  disappear 
exactly  equivalent  to  the  work  done.  The  weight  is  about 
one  pound,  and  to  raise  it  sixteen  feet  would  consume  as 
much  heat  as  would  raise  the  temperature  of  a  cubic  foot 
of  air  about  1°  F.  Conversely,  this  quantity  of  heat  Avould 
be  generated  by  the  falling  of  the  weight  from  a  height  of 
sixteen  feet.  It  is  easy  to  see  that,  if  the  force  of  gravity 
were  immensely  greater  than  it  is,  an  immensely  greater 
amount  of  heat  would  have  to  be  expended  to  raise  the 
weight.  The  greater  the  attraction,  the  greater  would  be 
the  amount  of  heat  necessary  to  overcome  it ;  but  conversely, 
the  greater  would  be  the  amount  of  heat  which  a  falling 
body  would  then  develope  by  its  collision  with  the  earth. 


ENEKGY  OF  MOLECULAR  FOECES.         155 

Having  made  our  minds  clear  that  heat  is  consumed 
when  a  weight  is  forcibly  separated  from  the  earth  by  this 
agent,  and  that  the  amount  of  heat  consumed  depends  on 
the  energy  of  the  attracting  force  overcome,  we  must  turn 
these  conceptions,  regarding  sensible  masses,  to  account, 
in  forming  conceptions  regarding  insensible  masses.  As 
an  intellectual  act  it  is  quite  as  easy  to  conceive  of  the  sep- 
aration of  two  mutually  attracting  atoms^  as  to  conceive  of 
the  separation  of  the  earth  and  weight.  I  have  already 
had  occasion  to  refer  more  than  once  to  the  energy  of 
molecular  forces,  and  here  I  have  to  return  to  the  subject. 
Closely  locked  together  as  they  are,  the  atoms  of  bodies, 
though  we  cannot  suppose  them  to  be  in  contact,  exert 
enormous  attractions.  It  would  require  an  almost  incred- 
ible amount  of  ordinary  mechanical  force  to  widen  the  dis- 
tances intervening  between  the  atoms  of  any  solid  or  liquid, 
so  as  to  increase  the  volume  of  the  solid  or  liquid  in  any 
considerable  degree.  It  would  also  require  a  force  of  great 
magnitude  to  squeeze  the  particles  of  a  liquid  or  solid  to- 
gether, so  as  to  make  the  body  less  in  size.  I  have  vainly 
tried  to  augment  the  density  of  a  soft  metal  by  pressure 
Water,  for  example,  which  yields  so  freely  to  the  hand 
plunged  in  it,  was  for  a  long  tune  regarded  as  absolutely 
incompressible.  Great  force  was  brought  to  bear  upon  it ; 
but  sooner  than  shrink  sensibly,  it  oozed  through  the  pores 
of  the  metal  vessel  which  contained  it,  and  spread  like  a 
dew  on  the  surface.*  By  refined  and  powerful  means  we 

*  I  have  to  thank  my  friend,  Mr.  Spedding,  for  the  following  extract 
in  reference  to  this  experiment : — 

*  Now  it  is  certain  that  rarer  bodies  (such  as  air)  allow  a  considerable 
degree  of  contraction,  as  has  been  stated ;  not  that  tangible  bodies  (such  as 
water)  suffer  compression  with  much  greater  difficulty  and  to  a  less  extent. 
How  far  they  do  suffer  it,  I  have  investigated  in.  the  following  experiment: 
I  had  a  hollow  globe  of  lead  made  capable,  of  holding  about  two  pints,  and 
sufficiently  thick  to  bear  considerable  force ;  having  made  a  hole  in  it,  I 


156  LECTTJEE   V. 

can  now  compress  water,  but  the  force  necessary  to  accom- 
plish this  is  very  great. 

When  we  wish  to  overcome  molecular  forces  we  must 
attack  them  by  their  peers.  Heat  accomplishes  what  me- 
chanical energy,  as  generally  wielded,  is  incompetent  to 
perform.  Bodies  when  heated  expand,  and  to  effect  this 
expansion  their  molecular  attractions  must  be  overcome. 
In  masses  equally  large  this  is  a  work,  in  comparison  with 
which  the  erection  of  the  Egyptian  pyramids  dwindles  to 
the  labour  of  mites  ;  and  where  the  attractions  to  be  over- 
come are  so  vast,  we  may  infer  that  the  quantity  of  heat 
necessary  to  overcome  them  will  be  commensurate. 

And  now  I  must  ask  your  entire  attention.     I  hold  in 

filled  it  with  water,  and  then  stopped  up  the  hole  with  melted  lead,  so  that 
the  globe  became  quite  solid.  I  then  flattened  the  two  opposite  sides  of  the 
globe  with  a  heavy  hammer,  by  which  the  water  was  necessarily  contracted 
into  less  space,  a  sphere  being  the  figure  of  largest  capacity ;  and  when  the 
hammering  had  no  more  effect  in  making  the  water  shrink,  I  made  use  of 
a  mill  or  press;  till  the  water,  impatient  of  further  pressure,  exuded 
through  the  solid  lead  like  a  fine  dew.  I  then  computed  the  space  lost  by 
the  compression,  and  concluded  that  this  was  the  extent  of  compression 
which  the  water  had  suffered,  but  only  when  constrained  by  great  violence.' 
(Bacon's  Novum  Organum  published  in  1620:  vol.  iv.  209  of  the  transla- 
tion.) Note  by  R.  Leslie  Ellis,  vol.  i.  p.  324. — This  is  perhaps  the  most 
remarkable  of  Bacon's  experiments,  and  it  is  singular  that  it  was  so  little 
spoken  of  by  subsequent  writers.  Nearly  fifty  years  after  the  production 
of  the  "  Novum  Organum,"  an  account  of  a  similar  experiment  was  publish- 
ed by  Megalotti,  who  was  secretary  of  the  Academia  del  Cimento  at  Flor- 
ence ;  and  it  has  since  been  familiarly  known  as  the  Florentine  experiment. 
I  quote  his  account  of  it,  "Facemmo  lavorar,"'  &c. 

The  writer  goes  on  to  remark  that  the  absolute  incompressibility  of 
water  is  not  proved  by  this  experiment,  but  merely  that  it  is  not  to  be  com- 
pressed in  the  manner  described  ;  but  the  experiment  is  on  other  grounds 
inconclusive. 

It  is  to  be  remembered  that  Leibnitz  ('  Nouveaux  Essais')  in  mentioning 
the  Florentine  experiment,  says  that  the  globe  was  of  gold  (p.  229  Erd- 
mann),  whereas  the  Florentine  academicians  expressly  say  why  they  pre- 
ferred silver  to  cither  gold  or  lead. 


INTEEIOK   WOKK.  157 

iny  hand  a  lump  of  lead ;  suppose  I  communicate  a  certain 
amount  of  heat  to  the  lead,  how  is  that  heat  disposed  of 
within  the  substance  ?  It  is  applied  to  two  distinct  pur- 
poses— it  performs  two  different  kinds  of  work.  One  por- 
tion of  it  imparts  that  species  of  motion  which  raises  the 
temperature  of  the  lead,  and  which  is  sensible  to  the  ther- 
mometer ;  but  another  portion  of  it  goes  to  force  the  atoms 
of  the  lead  into  new  positions,  and  this  portion  is  lost  as 
heat.  The  pushing  asunder  of  the  atoms  of  the  lead  in 
this  case,  in  opposition  to  their  mutual  attractions,  is  exact- 
ly analogous  to  the  raising  of  our  weight  in  opposition  to 
the  force  of  gravity.  Let  me  try  to  make  the  comparison 
between  the  two  actions  still  more  strict ;  suppose  that  I 
have  a  definite  amount  of  force  to  be  expended  on  our 
weight,  and  that  I  divide  this  force  into  two  portions,  one 
of  which  I  devote  to  the  actual  raising  of  the  weight,  while 
I  employ  the  other  to  cause  the  weight,  as  it  ascends,  to 
oscillate,  or  revolve,  like,  a  pendulum  or  governor,  and  to 
oscillate,  moreover,  with  gradually  augmented  energy  ;  we 
have,  then,  the  analogue  of  that  which  occurs  when  heat  is 
imparted  to  the  lead.  The  atoms  are  pushed  apart,  but 
during  their  recession  they  vibrate,  or  revolve,  with  grad- 
ually augmented  intensity.  Thus  the  heat  communicated 
to  the  lead  resolves  itself,  in  part,  into  atomic  potential 
energy,  and  in  part  into  a  kind  of  atomic  music,  the  music- 
al part  alone  being  competent  to  act  upon  our  thermome- 
ters or  to  affect  our  nerves. 

In  this  case,  then,  the  heat  accomplishes  what  we  may 
call  interior  worlc  ;*  it  performs  work  within  the  body 
heated,  by  forcing  its  particles  to  take  up  new  positions. 
When  the  body  cools,  the  forces  which  were  overcome  in 
the  process  of  heating  come  into  play,  and  the  heat  which 
was  consumed  by  the  forcing  asunder  of  the  atoms  is  now 
restored  by  the  drawing  together  of  the  atoms. 

*  See  the  excellent  memoirs  of  Clausius  in  the  Philosophical  Magazine. 


158  ^ECTUKE   V. 


Chemists  have  determined  the  relative  weights  of  the 
atoms  of  different  substances.  Calling  the  weight  of  a  hy- 
drogen atom  1,  the  weight  of  an  oxygen  atom,  you  know, 
is  16.  Hence  to  make  up  a  pound  weight  of  hydrogen, 
sixteen  times  the  number  of  atoms  contained  in  a  pound 
of  oxygen  would  be  necessary.  The  number  of  atoms  re- 
quired to  make  up  a  pound  is  evidently  inversely  propor- 
tional to  the  atomic  weight.  We  here  approach  a  very 
delicate  and  important  point.  The  experiments  of  Dulong 
and  Petit,  and  of  MM.  Regnault  and  Neumann,  render  it 
extremely  probable  that  all  elementary  atoms,  great  and 
small,  light  and  heavy,  when  at  the  same  temperature, 
possess  the  same  amount  of  the  energy  which  we  call  heat, 
the  lighter  atoms  making  good  by  velocity  what  they  want 
in  mass.  Thus,  each  of  the  atoms  of  hydrogen  has  the 
same  moving  energy  as  an  atom  of  oxygen  at  the  same 
temperature.  But,  inasmuch  as  a  pound  weight  of  hydro- 
gen contains  sixteen  -times  the  number  of  atoms,  it  must 
also  contain  sixteen  times  the  amount  of  heat  possessed  by 
a  pound  of  oxygen,  at  the  same  temperature. 

From  this  it  follows  that  to  raise  a  pound  of  hydrogen, 
a  certain  number  of  degrees  in  temperature  —  say  from  50° 
to  60°  —  would  require  sixteen  times  the  amount  of  heat 
needed  by  a  pound  of  oxygen  under  the  same  circum- 
stances. Conversely,  a  pound  of  hydrogen,  in  falling 
througli  10°,  would  yield  sixteen  times  the  amount  of  heat 
yielded  by  a  pound  of  oxygen,  in  falling  through  the  same 
number  of  degrees. 

In  oxygen  and  hydrogen  we  have  no  sensible  amount 
of  '  interior  work,'  to  be  performed  ;  there  are  no  molecu- 
lar attractions  of  sensible  magnitude  to  be  overcome.  But 
in  solid  and  liquid  bodies,  besides  the  differences  due  to 
the  number  of  atoms  present  in  the  unit  of  weight,  we  have 
also  differences  due  to  the  consumption  of  heat  in  interior 
work.  Hence  it  is  clear  that  the  amount  of  heat  which 


RELATIONS   OF   ATOMIC  NUMBERS   TO 


159 


Fig.  43. 


different  bodies  contain  is  not  at  all  declared  by  their  tem- 
perature. To  raise  a  pound  of  water,  for  example,  1°, 
would  require  thirty  times  the  amount  of  heat  necessary  to 
raise  a  pound  of  mercury  1°.  Conversely,  the  pound  of 
water,  in  falling  through  1°,  would  yield  up  thirty  times 
the  amount  of  heat  yielded  up  by  the  pound  of  mercury. 

Let  me  illustrate,  by  a  simple  experiment,  the  differ- 
ences which  exist  between  bodies,  as  to  the  quantity  of 
heat  which  they  contain.  I  have  here  a  cake  of  beeswax 
six  inches  in  diameter  and  half  an  inch  thick.  Here  I  have 
a  vessel  containing  oil,  which  is  now  at  a  temperature  of 
180°  C.  In  the  hot  oil  I  have  immersed  a  number  of  balls 
of  different  metals — of  iron,  lead,  bismuth,  tin  and  copper. 
At  present  they  all  possess  the  same  temperature,  namely, 
that  of  the  oil.  Well,  I  lift  them  out  of  the  oil,  and  place 
them  upon  this  cake  of  wax  c  D 
(fig.  43),  which  is  supported  by 
the  ring  of  a  retort-stand  ;  they 
melt  the  wax  underneath  and 
sink  in  it.  But  I  see  that  they 
are  sinking  with  different  ve- 
locities. The  iron  and  the  cop- 
per are  working  themselves 
much  more  vigorously  into  the 
fusible  mass  than  the  others ;  the 
tin  comes  next,  while  the  lead 
and  the  bismuth  lag  entirely  be- 
hind. There  goes  the  iron  clean  - 
through,  the  copper  follows ;  I 
can  see  the  bottom  of  the  tin 

ball  just  peeping  through  the  lower  surface  of  the  cake, 
but  it  cannot  go  farther  ;  while  the  lead  and  bismuth  have 
made  but  little  way,  being  unable  to  sink  to  much  more 
than  half  the  depth  of  the  cake. 

Supposing,  then,  I  take  equal  weights  of  different  sub- 


1GO 


LECTURE   V. 


stances,  heat  them  all  (say  to  100°)  and  then  determine  the 
exact  amount  of  heat  which  each  of  them  gives  out  in 
cooling  from  100°  to  0°,  I  should  find  very  different 
amounts  of  heat  for  the  different  substances.  How  could 
this  problem  be  solved  ?  It  has  been  solved  by  eminent 
men  by  observing  the  time  which  a  body  requires  to  cool. 
Of  course  the  greater  the  amount  of  heat  possessed  and 
generated  by  its  atoms,  the  longer  would  the  body  take  to 
cool.  The  relative  quantities  of  heat  yielded  up  by  differ- 
ent bodies  have  also  been  determined  by  plunging  them, 
when  heated,  into  cold  water,  and  observing  the  gain  on 
the  one  hand  and  the  loss  on  the  other.  The  problem  has 
also  been  solved  by  observing  the  quantities  of  ice  which 
different  bodies  can  liquefy,  in  falling  from  212°  Fahr.  to 
32°,  or  from  100°  C.  to  0°.  These  different  methods  have 
given  concordant  results.  According  to  the  celebrated 
Krench  experimenter  Regnault,  the  following  numbers 
express  the  relative  amounts  of  heat  given  out  by  a  unit 
of  weight  of  each  of  the  substances  the  names  of  which 
are  annexed,  in  cooling  from  98°  C.  to  15°  C. 


Aluminium     . 

0-2143 

Antimony 

0-0508 

Arsenic 

0-0814 

Bismuth 

0-0308 

Boron  . 

0-2352 

Bromine 

0-1129 

Cadmium 

0-0567 

Carbon 

0-2414 

Lithium 

0-9408 

Magnesium     . 

0-2499 

Manganese      . 

0-1217 

Mercury 

0-0333 

Nickel 

/H086 

Osmium 

0-0311 

Palladium 

0-0593 

Phosphorus  (solid) 

0-1887 

"    (amorphous) 

01700 

Platinum 

0-0329 

Potassium 

0-1696 

Ehodium 

0-0580 

Cobalt 

0-1067 

Copper 

0-0952-.' 

Diamond  • 

0-1469 

Gold 

0-0324 

Iodine 

0-0541 

Iridium 

0-0326 

Iron 

0-1138  « 

Lead 

0-0314 

Selenium  . 

0-0827 

Silicon 

0-1774 

Silver 

0-0570 

Sodium 

0-2934 

Sulphur  (native)  . 

01776 

"    (recently  melted) 

0-2026 

Tellurium 

0-0474 

Thallium  . 

0-0336 

Tin 

0-0562 

Tungsten  . 

0-0334 

Water 

1-0080 

Zinc 

0-0955 

A  moment's  inspection  of  this  table  explains  the  reason 


MECHANICAL   VALUE   OF   SPECIFIC   HEAT.  161 

why  the  iron  and  copper  balls  melted  through  the  wax, 
while  the  lead  and  bismuth  balls  were  incompetent  to  do 
so ;  it  will  also  be  seen  that  tin  here  occupies  the  position 
which  we  should  assign  to  it,  after  our  experiment  with 
the  cake  of  wax ;  water,  we  see,  yields  more  heat  than 
any  other  substance  in  the  list. 

Each  of  these  numbers  denotes  what  has  been  hitherto 
called  the  '  specific  heat,'  or  the  {  capacity  for  heat,'  of  the 
substance  to  which  it  is  attached.  As  stated  on  a  former 
occasion,  those  who  considered  heat  to  be  a  fluid,  ex- 
plained these  differences  by  saying  that  some  substances 
had  a  greater  store  of  this  fluid  than  others.  We  may, 
without  harm,  continue  to  use  the  term  *  specific  heat,'  or 
'  capacity  for  heat,'  now  that  we  know  the  true  nature  of 
the  actions  denoted  by  the  term.  It  is  a  noteworthy  fact, 
that  as  the  specific  heat  increases,  the  atomic  weight  di- 
minishes, and  vice  versd  ;  so  that  the  product  of  the  atomic 
weight  and  specific  heat  is,  in  almost  all  cases,  a  sensibly 
constant  quantity.  This  illustrates  a  remark  already 
made,  that  the  lighter  atoms  make  good  by  velocity  what 
they  want  in  mass.  * 

The  magnitude  of  the  forces  engaged  in  this  automic 
motion,  and  interior  work,  as  measured  by  any  ordinary 
mechanical  standard,  is  enormous.  I  have  here  a  pound 
of  iron,  which,  on  being  heated  from  0°  C.  to  100°  C.  ex- 
pands by  about  -g-^th  of  the  volume  which  it  possesses 
at  0°.  Its  augmentation  of  volume  would  certainly  escape 
the  most  acute  eye ;  still,  to  give  its  atoms  the  motion 
corresponding  to  this  augmentation  of  temperature,  and 
to  shift  them  through  the  small  space  indicated,  an  amount 
of  heat  is  requisite  which  would  raise  about  eight  tons 
one  foot  high.  The  force  of  gravity  almost  vanishes  in 
comparison  with  these  molecular  forces ;  the  pull  of  the 
earth  upon  the  pound  weight,  as  a  mass,  is  as  nothing 
compared  with  the  mutual  pull  of  its  own  molecules. 


162  LECTURE  V. 

Water  furnishes  a  still  subtler  example.  Water  expands 
on  both  sides  of  4°  C.  or  39°  F. ;  at  4°  C.  it  has  its  maxi- 
mum density.  Suppose  a  pound  of  water  to  be  heated 
from  3^°  C.  to  4-J°  C. — that  is,  one  degree — its  volume 
at  both  temperatures  is  the  same;  there  has  been  no 
forcing  asunder  whatever  of  the  atomic  centres,  and  still, 
though  the  volume  is  unchanged,  an  amount  of  heat  has 
been  imparted  to  the  water,  sufficient,  if  mechanically 
applied,  to  raise  a  weight  of  1,390  Ibs.  a  foot  high.  The 
interior  work,  done  here  by  the  heat,  is  simply  that  of 
causing  the  atoms  of  water  to  rotate.  It  separates  the 
attracting  poles  of  the  atoms  by  a  tangential  movement, 
but  leaves  their  centres  at  the  same  distance  asunder,  first 
and  last.  The  conceptions  with  which  I  here  deal  may 
not  be  easy  to  those  unaccustomed  to  such  studies,  but 
they  can  be  realized,  with  perfect  clearness,  by  all  who 
have  the  patience  to  dwell  upon  them  for  a  sufficient 
length  of  time. 

Here  we  may  note  further,  that  there  are  descriptions 
of  interior  work,  different  from  that  of  pushing  the  atoms 
more  widely  apart.  An  enormous  quantity  of  interior 
work  may  be  accomplished,  while  the  atomic  centres,  in- 
stead of  being  pushed  apart,  approach  each  other.  Polar 
forces — forces  emanating  from  distinct  atomic  points,  and 
acting  in  distinct  directions,  give  to  crystals  their  sym- 
metry, and  the  overcoming  of  these  forces,  while  it  neces- 
sitates a  consumption  of  heat,  may  also  be  accompanied 
by  a  diminution  of  volume.  This  is  illustrated  by  the 
deportment  of  both  ice  and  bismuth  in  liquefying. 

The  most  important  experiments  on  the  specific  heat 
of  elastic  fluids  we  owe  to  M.  Kegnault.  He  determined 
the  quantities  of  heat  necessary  to  raise  equal  weights  of 
gases  and  vapours,  and  also  the  quantities  necessary  to 
raise  equal  volumes  of  them,  through  the  same  number  of 
degrees.  Calling  the  specific  heat  of  water  1,  here  are 
some  of  the  results  of  this  invaluable  investigation  : — 


SPECIFIC   HEAT   OF   GASES.  163 

SIMPLE  GASES. 

Specific  heats 


Equal  weights  Equal  volumes 

Air  .  .  .  0-237 

Oxygen  .  .  .  0*218  0'240 

Nitrogen  .  .  .  0'244  0-237 

Hydrogen  .  .  .  3  "409  0'23G 

Chlorine  .  .  .  0-121  0'296 

Bromine  .  .  .  0'055  0'304 

We  have  already  arrived  at  the  conclusion  that,  for 
equal  weights,  hydrogen  would  be  found  to  possess  six- 
teen times  the  amount  of  heat  possessed  by  oxygen,  and 
fourteen  times  that  of  nitrogen,  because  hydrogen  consists 
of  sixteen  times  the  number  of  atoms,  in  the  one  case,  and 
fourteen  times  the  number,  in  the  other.  Now,  we  find 
this  conclusion  verified  experimentally.  Equal  volumes, 
moreover,  of  all  these  gases  contain  the  same  number  of 
atoms,  and  hence  we  should  infer  that  the  specific  heats 
of  equal  volumes  ought  to  be  equal.  They  are  very 
nearly  so  for  oxygen,  nitrogen,  and  hydrogen ;  but  chlo- 
rine and  bromine  differ  considerably  from  the  other  ele- 
mentary gases.  Now  bromine  is  a  vapour,  and  chlorine  a 
gas,  easily  liquefied  by  pressure;  hence,  in  both  these 
cases,  the  mutual  attraction  of  the  atoms,  which  is  insen- 
sible in  oxygen,  nitrogen,  and  hydrogen,  requires  a  por- 
tion of  heat  to  overcome  it.  The  specific  heats  of  chlorine 
and  bromine  at  equal  volumes  are,  therefore,  higher. 

Certain  simple  gases  unite  to  form  compound  ones, 
without  any  change  of  volume.  Thus,  one  volume  of 
chlorine  combines  with  one  volume  of  hydrogen,  to  form 
two  volumes  of  hydrochloric  acid.  In  other  cases  the  act 
of  combination  is  accompanied  by  a  diminution  of  vol- 
ume ;  thus,  two  volumes  of  nitrogen  combine  with  one  of 
oxygen  to  form  two  volumes  of  the  protoxide  of  nitrogen. 
By  the  act  of  combination,  three  volumes  have,  in  this 
case,  been  condensed  to  two.  M.  Regnault  finds  that  the 
compound  gases  which  do  not  change  volume,  have,  at 


164:  LECTUEE  V. 

equal  volumes,  the  same  specific  heat  as  oxygen,  nitrogen, 
and  hydrogen ;  while  with  those  which  change  volume, 
this  is  not  the  case. 

COMPOUND  GASES — WITHOUT  CONDENSATION. 

Specific  heats 


Equal  weights  Equal  volumes 
Nitric  oxide         .  .  .     Q'232  0'241 

Carbonic  oxide  .  .  .     0*245  0-237 

Hydrochloric  acid  .  .     0-185  0-235 

The  specific  heat  of  equal  volumes  of  these  compound 
gases  is  the  same  as  that  of  the  three  simple  gases  already 
mentioned. 

COMPOUND  GASES — 3  VOLUMES  CONDENSED  TO  2. 

Specific  heats 

Equal  weights  Equal  volumes 

Carbonic  acid     .  .  .  0*217  0-331 

Nitrous  oxide      .  .  .  0*226  0-345 

Aqueous  vapour  .  .  0-480  0-299 

Sulphurous  acid  .  .  0-154  0'341 

Sulphide  of  hydrogen,  .  .  0.243  0-286 

Bisulphide  of  carbon,  .  .  0-157  0'412 

Here  we  find  the  specific  heats  of  equal  volumes  nei- 
ther equal  to  those  of  the  elementary  gases,  nor  equal  to 
each  other.  It  is  worth  bearing  in  mind  that  the  specific 
heat  of  water  is  about  double  that  of  aqueous  vapour,  and 
also  double  that  of  ice. 

The  high  specific  heat  of  water  has  one  important 
bearing  which  I  do  not  wish  to  pass  over  here.  Compar- 
ing equal  weights,  the  specific  heat  of  water  being  1,  that 
of  air  is  0'237.  Hence,  a  pound  of  water,  in  losing  one 
degree  of  temperature,  would  warm  about  4*2  Ibs.  of  air 
one  degree.  But  water  is  770  times  heavier  than  air; 
hence,  comparing  equal  volumes^  a  cubic  foot  of  water,  in 
losing  one  degree  of  temperature,  would  raise  770x4'2  = 
3,234  cubic  feet  of  air,  one  degree. 

The  vast  influence  which,  the  ocean  must  exert,  as  a 
moderator  of  climate,  here  suggests  itself.  The  heat  of 


THE   OCEAN   A  MODEKATOK   OF   CLIMATE.  165 

summer  is  stored  up  in  the  ocean,  and  slowly  given  out 
during  the  winter.  Hence  one  cause  of  the  absence  of 
extremes  in  an  island  climate.  The  summers  of  the  island 
can  never  attain  the  fervid  heat  of  the  continental  summer, 
nor  can  the  winter  of  the  island  be  so  severe  as  the  conti- 
nental winter.  In  various  parts  of  the  continent  fruits 
grow  which  our  summers  cannot  ripen ;  but  in  these  same 
parts  our  evergreens  are  unknown ;  for  they  cannot  live 
through  the  winters.  The  winter  of  Iceland  is,  as  a  gen- 
eral rule,  milder  than  that  of  Lombardy. 

We  have  hitherto  confined  our  attention  to  tne  neat 
consumed  in  the  molecular  changes  of  solid  and  liquid 
bodies  while  these  bodies  continue  solid  and  liquid.  We 
shall  now  direct  our  attention  to  the  phenomena  which  ac- 
company changes  of  the  state  of  aggregation.  When  suffi- 
ciently heated,  a  solid  melts,  and  when  sufficiently  heated, 
a  liquid  assumes  the  form  of  gas.  Let  us  take  the  case  of 
ice,  and  trace  it  through  the  entire  cycle.  This  block  of 
ice  has  now  a  temperature  of  20°  F.  I  warm  it ;  a  ther- 
mometer fixed  in  it  rises  to  32°,  and  at  this  point  the  ice 
begins  to  melt ;  the  thermometric  column,  which  rose  pre- 
viously, is  now  arrested  in  its  march,  and  becomes  perfectly 
stationary.  I  continue  to  apply  warmth,  but  there  is  no 
augmentation  of  temperature  ;  and  not  till  all  the  solid  has 
been  reduced  to  liquid  does  the  thermometer  resume  its 
motion.  It  is  now  again  ascending  ;  it  reaches  100°,  200°, 
212°:  here  steam-bubbles  show  themselves  in  the  liquid; 
it  boils,  and  from  this  point  onwards  the  thermometer  re- 
mains stationary  at  212°. 

But  during  the  melting  of  the  ice  and  during  the  evap- 
oration of  the  water,  heat  is  incessantly  communicated  :  to 
simply  liquefy  the  ice,  as  much  heat  has  been  imparted  to 
it  as  would  raise  the  same  weight  of  water  143°  Fahr.,  or 
as  would  raise  143  times  the  weight  1°  F.  in  temperature ; 
and  to  convert  a  pound  of  water  at  212°  into  a  pound  of 


1G6  LECTURE  V. 

steam  at  the  same  temperature,  967  times  as  much  heat  is 
required  as  would  raise  a  pound  of  water  1°  in  temperature. 
The  former  number,  143°,  represents  what  has  been  hither- 
to called  the  latent  heat  of  water ;  and  the  latter  number, 
967°,  represents  the  latent  heat  of  steam.  It  was  manifest 
to  those  who  first  used  these  terms,  that,  throughout  the 
entire  time  of  melting,  and  throughout  the  entire  time  of 
boiling,  heat  was  communicated ;  but  inasmuch  as  this  heat 
was  not  revealed  by  the  thermometer,  the  fiction  was  in- 
vented that  it  was  rendered  latent.  The  fluid  of  heat  hid 
itself  in  some  unknown  way  in  the  interstitial  spaces  of  the 
water  and  of  the  steam.  According  to  our  present  theory, 
the  heat  expended  in  melting  is  consumed  in  conferring 
potential  energy  upon  the  atoms.  It  is  virtually  the  lifting 
of  a  weight.  So  likewise  as  regards  the  steam,  the  heat  is 
consumed  in  pulling  the  liquid  molecules  asunder,  confer- 
ring upon  them  a  still  greater  amount  of  potential  energy  ; 
and  when  the  heat  is  withdrawn,  the  vapour  condenses  and 
the  molecules  again  clash  with  a  dynamic  energy  equal  to 
that  which  was  employed  to  separate  them,  and  the  precise 
quantity  of  heat  then  consumed  now  reappears. 

The  act  of  liquefaction  consists  of  interior  work  ex- 
pended in  moving  the  atoms  into  new  positions.  The  act 
of  vaporisation  is  also,  for  the  most  part,  interior  work ;  to 
which  however  must  be  added  the  external  work  performed 
in  the  expansion  of  the  vapour,  which  makes  place  for  it- 
self by  forcing  back  the  atmosphere. 

We  are  indebted  to  the  eminent  man  to  whom  I  have 
referred  so  often,  for  the  first  accurate  determinations  of 
the  calorific  power  of  fuel.  '  Rumford  estimated  the  cal- 
orific power  of  a  body  by  the  number  of  parts,  by  weight, 
of  water,  which  one  part,  by  weight,  of  the  body  would, 
on  perfect  combustion,  raise  1°  in  temperature.  Thus  one 
part,  by  weight,  of  charcoal,  in  combining  with  2f  parts 
of  oxygen  to  form  carbonic  acid,  will  evolve  heat  sufficient 


LATENT   HEAT   OF  LIQUIDS.  167 

to  raise  the  temperature  of  about  8,000  parts  by  weight  of 
water  1°  C.  Similarly,  one  pound  of  hydrogen,  in  com- 
bining with  eight  pounds  of  oxygen  to  form  water,  will 
raise  34,000  Ibs.  of  water  1°  C.  The  relative  calorific  pow- 
ers, therefore,  of  carbon  and  hydrogen  are  as  8  :  34.'* 
The  recent  refined  researches  of  Favre  and  Silbermann  en- 
tirely confirm  the  determinations  of  Rumford. 

Let  us,  then,  fix  our  attention  upon  this  wonderful  sub- 
stance, water,  and  trace  it  through  the  various  stages  of 
its  existence.  First  we  have  its  constituents  as  free  atoms, 
which  attract  each  other,  fall,  and  clash  together.  The 
mechanical  value  of  this  atomic  act  is  easily  determined ; 
knowing  the  number  of  foot-pounds  corresponding  to  the 
heating  of  1  Ib.  of  water  1°  C.,  we  can  readily  calculate  the 
number  of  foot-pounds  equivalent  to  the  heating  of  34,000 
Ibs.  of  water  1°  C.  Multiplying  the  latter  number  by 
l,390,f  we  find  that  the  concussion  of  our  1  Ib.  of  hydrogen 
wth  8  Ibs.  of  oxygen  is  equal,  in  mechanical  value,  to  the 
raising  of  forty-seven  million  pounds  one  foot  high !  I 
think  I  did  not  overrate  matters  when  I  said  that  the  force 
of  gravity,  as  exerted  near  the  earth,  was  almost  a  vanish- 
ing quantity,  in  comparison  with  these  molecular  forces  ; 
and  bear  in  mind  the  distances  which  separate  the  atoms 
before  combination — distances  so  small  as  to  be  utterly 
immeasurable  ;  still  it  is  in  passing  over  these  distances 
that  the  atoms  acquire  a  velocity  sufficient  to  cause  them 
to  clash  with  the  tremendous  energy  indicated  by  the  above 
numbers. 

After  combination  the  substance  is  in  a  state  of  vapour, 
which  sinks  to  212°,  and  afterwards  condenses  to  water. 
In  the  first  instance  the  atoms  fell  together  to  form  the 
compound  ;  in  the  next  instance  the  molecules  of  the  com- 

*  Percy's  Metallurgy,  p.  53. 

|  772  foot-pounds  being  the  mechanical  equivalent  for  1°  F.,  1,390 
foot-pounds  is  the  equivalent  for  1°  C. 


1G8  LECTURE   V. 

pound  fall  together  to  form  a  liquid.  The  mechanical  val- 
ue of  this  act  is  also  easily  calculated :  9  Ibs.  of  steam  in 
falling  to  water,  generate  an  amount  of  heat  sufficient  to 
raise  967  X  9  =  8,703  Ibs.  of  water  1°  F.  Multiplying 
this  number  by  772,  we  have  a  product  of  6,718,716  foot- 
pounds as  the  mechanical  value  of  the  mere  act  of  conden- 
sation.* The  next  great  fall  of  our  9  Ibs.  of  water  is  from 
the  state  of  liquid  to  that  of  ice,  and  the  mechanical  value 
of  this  act  is  equal  to  993,564  foot-pounds.  Thus  our  9 
Ibs.  of  water,  in  its  origin  and  progress,  falls  down  three 
great  precipices  :  the  first  fall  is  equivalent  to  the  descent 
of  a  ton  weight  urged  by  gravity  down  a  precipice  22,320 
feet  high ;  the  second  fall  is  equal  to  that  of  a  ton  down 
a  precipice  2,900  feet  high ;  and  the  third  is  equal  to  the 
descent  of  a  ton  down  a  precipice  433  feet  high.  I 
have  seen  the  wild  stone-avalanches  of  the  Alps,  which 
smoke  and  thunder  down  the  declivities  with  a  vehemence 
almost  sufficient  to  stun  the  observer.  I  have  also  seen 
snow-flakes  descending  so  softly  as  not  to  hurt  the  fragile 
spangles  of  which  they  were  composed ;  yet  to  produce, 
from  aqueous  vapour,  a  quantity  of  that  tender  material 
which  a  child  could  carry,  demands  an  exertion  of  energy 
competent  to  gather  up  the  shattered  blocks  of  the  largest 
stone-avalanche  I  have  ever  seen,  and  pitch  them,  to  twice 
the  height  from  which  they  fell. 

I  will  now  relieve  the  strain  which  I  have  hitherto  put 
upon  your  attention,  by  introducing  a  few  experimental 
illustrations  of  the  calorific  effects  which  accompany  the 
change  of  aggregation.  I  place  my  thermo-electric  pile 
thus  upon  its  back  on  the  table,  and  on  its  naked  face  I 

*  In  Rumford's  experiments  the  heat  of  condensation  was  included  in 
his  estimate  of  calorific  power ;  deducting  the  above  number  from  that 
found  for  the  chemical  union  of  the  hydrogen  and  oxygen,  forty  millions  of 
foot-pounds  would  still  remain  as  the  mechanical  value  of  the  act  of  com' 
bination. 


EXPERIMENTAL   ILLUSTRATIONS.  169 

place  this  thin  silver  basin,  B  (fig.  44),  into  which  I  pour  a 
quantity  of  water  slightly  warmed,  the  needle  of  the  gal- 
vanometer moves  to  90°,  and  remains  permanently  deflected 
to  70°.  I  now  place  a  little  powdered  nitre,  not  more  than 
can  fit  upon  a  three-penny  piece,  in  the  basin,  and  allow  it 
to  dissolve.  I  had  placed  the  nitre  previously  before  the 
fire,  so  that  not  only  was  the  liquid  warm,  but  the  solid 
powder  was  also  warm.  Observe  the  effect  of  their  mix- 
Fig.  44. 


ture !  The  nitre  dissolves  in  the  water ;  and  to  produce 
this  change,  all  the  heat  which  both  the  water  and  the  nitre 
possess,  in  excess  of  the  temperature  of  this  room,  is  con- 
sumed, and,  indeed,  a  great  deal  more.  The  needle,  you 
see,  sinks  not  only  to  zero,  but  goes  strongly  up  at  the 
other  side,  showing  that  now  the  face  of  the  pile  is  power- 
fully chilled. 

I  remove  the  basin,  pour  the  liquid  out,  and  resupply  it 
with  warm  water,  into  which  I  introduce  a  pinch  of  com- 
mon salt.  The  needle  was  at  70°  when  the  salt  was  intro- 
duced :  it?  is  now  sinking,  reaches  zero,  and  goes  up  on  the 
side  which  indicates  cold.  But  the  action  is  not  at  all  so 
strong  as  in  the  case  of  saltpetre.  The  reason  is  that  the 
amount  of  interior  work  required  by  the  salt,  and  which 
necessitates  the  consumption  of  heat,  is  much  less  than  that 
demanded  by  the  nitre.  As  regards  latent  heat,  then,  we 
have  differences  similar  to  those  which  we  have  already 
illustrated  as  regards  specific  heat.  Again,  I  cleanse  the 
basin,  put  fresh  water  in  it,  and  put  a  little  sugar  in  the 
water  ;  the  amount  of  heat  absorbed  in  the  solution  of  the 
8 


170 


LECTURE   V. 


sugar  is  sensible,  the  liquid  is  chilled,  but  the  amount  of 
chilling  is  much  less  than  in  either  of  the  former  cases. 
Thus,  wfren  you  sweeten  your  hot  tea,  you  cool  it  in  the 
most  philosophical  manner ;  when  you  put  salt  in  your 
soup,  you  do  the  same ;  and  if  you  were  concerned  with  the 
act  of  cooling  alone,  and  careless  of  the  flavour  of  your 
soup,  you  might  hasten  its  refrigeration  by  adding  saltpetre. 
In  a  former  lecture  I  made  use  of  a  mixture  of  pounded 
ice  and  salt  to  obtain  great  cold.  Both  the  salt  and  the  ice 
when  they  are  thus  mixed  together,  change  their  state  of 
aggregation ;  the  amount  of  interior  work  is  here  so  great, 
that  during  its  performance  the  temperature  of  the  mix- 
ture sinks  30°  Fahr.,  and  more,  below  the  freezing  point 
of  water.  Here  is  a  nest  of  watch-glasses  which  I  have 
wrapped  in  tinfoil,  and  immersed  in  a  mixture  of  ice  and  salt. 
Into  each  watch-glass  I  had  poured  a  little  water,  in  which 
the  next  glass  rested.  They  are  now  all  frozen  together  to 
a  solid  cylinder,  by  the  cold  of  this  mixture  of  ice  and  salt. 
I  will  now  reverse  the  process,  and  endeavour  to  show 
you  the  heat  developed  in  pass- 
ing from  the  liquid  to  the  solid 
state.  But  first  let  me  show  you 
that  heat  is  rendered  latent  when 
sulphate  of  soda  is  dissolved.  I 
experiment  with  the  substance 
exactly  as  I  experimented  with 
the  nitre,  and  you  see,  that  as  the 
crystals  melt  in  the  water  the  pile 
is  chilled.  Arid  now  for  the  com- 
plementary experiment.  This 
large  glass  bolt-head  B  (fig.  45), 
with  this  long  neck,  is  now  filled 
with  a  solution  of  sulphate  of  so- 
da. Yesterday  Mr.  Anderson 
dissolved  the  substance  in  a  pan 


HEAT  ACCOMPANYING   SOLIDIFICATION.  171 

over  our  laboratory  fire,  and  filled  this  bolt-head  with  the 
solution.  He  then  covered  the  top  carefully  with  a  piece 
of  bladder,  and  placed  the  bottle  behind  this  table,  where 
it  has  remained  undisturbed  throughout  the  night. 

The  liquid  is,  at  the  present  moment,  supersaturated 
with  sulphate  of  soda.  When  the  water  was  hot,  it  melted 
more  than  it  could  melt  when  cold.  But  now  the  tempera- 
ture has  sunk  much  lower  than  that  which  corresponds  to 
the  point  of  saturation.  This  state  of  things  is  secured  by 
keeping  the  solution  perfectly  still,  and  permitting  nothing 
to  fall  into  it.  Water,  kept  thus  still,  may  be  cooled  many 
degrees  below  its  freezing  point.  Some  of  you  may  have 
noticed  the  water  in  your  jugs,  after  a  cold  winter  night, 
suddenly  freeze  on  being  poured  out  in  the  morning.  In 
cold  climates  this  is  not  uncommon.  Well,  the  particles  of 
sulphate  of  soda  in  this  solution  are  on  the  brink  of  a  preci- 
pice, and  I  can  push  them  over  it,  by  simply  dropping  a 
small  crystal  of  the  substance,  not  larger  than  a  grain  of 
sand,  into  the  solution.  Observe  what  takes  place ;  the 
bottle  now  contains  a  clear  liquid  ;  I  drop  the  bit  of  crys- 
tal in,  it  does  not  sink  ;  the  molecules  have  closed  round  it 
to  form  a  solid  in  which  it  is  now  embedded.  The  passage 
of  the  atoms  from  a  state  of  freedom  to  a  state  of  bondage 
goes  on  quite  gradually ;  you  see  the  solidification  extend- 
ing down  the  neck  of  the  bottle.  Observe  where  I  have 
placed  my  thermo-electric  pile  P.  Its  naked  face  rests 
against  the  convex  surface  of  the  bottle,  and  the  needle  of 
the  galvanometer  points  to  zero.  The  process  of  crystalli- 
sation has  not  yet  reached  the  liquid  in  front  of  the  pile, 
but  you  see  it  approaching.  It  is  now  solidified  opposite 
the  pile,  and  mark  the  eifect.  The  atoms,  in  falling  to  the 
solid  form,  develope  heat ;  this  heat  communicates  itself  to 
the  glass  envelope,  the  glass  envelope  warms  the  pile,  and 
the  needle,  as  you  see,  flies  to  90°.  The  quantity  of  heat 


172  LECTUKE   V. 

thus  rendered  sensible  by  solidification  is  exactly  equal  to 
that  which  was  rendered  latent  by  liquefaction. 

We  have,  in  these  experiments,  dealt  with  the  latent 
heat  of  liquids  ;  let  me  now  direct  your  attention  to  a  few 
experiments  illustrative  of  what  has  been  called  the  latent 
heat  of  vapours — in  other  words,  the  heat  consumed  in 
conferring  potential  energy,  when  a  body  passes  from  the 
liquid  to  the  gaseous  state.  As  before,  I  turn  my  pile  upon 
its  back  with  its  naked  face  upwards,  and  on  this  face  I 
place  the  silver  basin  already  used,  into  which  I  have 
poured  a  small  quantity  of  a  volatile  liquid,  which  I  have 
purposely  warmed.  The  needle  now  moves,  indicating 
heat.  But  scarcely  has  it  attained  90°  when  it  turns 
promptly,  descends  to  0°,  and  flies  with  violence  up  on  the 
side  of  cold.  The  liquid  here  used  is  sulphuric  ether  ;  it  is 
very  volatile,  and  the  speed  of  its  evaporation  is  such  that 
it  consumes,  rapidly,  the  heat  at  first  communicated  to  it, 
and  then  abstracts  heat  from  the  face  of  the  pile.  I  re- 
move the  ether,  and  supply  its  place  by  alcohol,  slightly 
warm ;  the  needle,  as  before,  goes  up  on  the  side  of  heat. 
But  wait  a  moment ;  I  will  use  these  small  bellows  to  pro- 
mote the  evaporation  of  the  alcohol ;  now  you  see  the  nee- 
dle descending,  and  now  it  is  up  at  90°  on  the  side  of  cold. 
Water  is  not  nearly  so  volatile  as  alcohol,  still  I  can  show 
the  absorption  of  heat  by  the  evaporation  of  water  also. 
We  use  a  kind  of  pottery  for  holding  water,  which  admits 
of  a  slight  percolation  of  the  liquid,  so  as  to  cause  a  kind 
of  dewiness  on  the  external  surface.  Evaporation  goes  on 
from  that  surface,  and  the  heat  necessary  to  this  work, 
being  drawn  in  great  part  from  the  water  within,  keeps  it 
cool.  Butter-coolers  are  made  on  the  same  principle. 

To  show  you  the  extent  to  which  refrigeration  may  be 
carried  by  the  evaporation  of  water,  I  have  here  an  instru- 
ment (fig.  46),  by  which  water  is  frozen,  through  the  sim- 
ple abstraction  of  its  heat  by  its  own  vapour.  The  instru- 


LATENT  HEAT  OF  VAPORS.  173 

ment  is  called  the  cryophorus,  or  ice-carrier,  and  it  was 
invented  by  Dr.  Wollaston.  It  is  made  in  this  way — a  lit- 
tle water  is  put  into  one  of  these  bulbs  ;  the  other  bulb,  B, 
when  softened  by  heat,  had  a  tube  drawn  out  from  it  with 
a  minute  aperture  at  the  end.  Well,  the  water  was  boiled 
in  A,  and  steam  was  produced,  until  it  had  chased  all  the 
air  away  through  the  small  aperture  in  the  distant  bulb. 
When  the  bulbs  and  connecting  tube  were  filled  with  pure 
steam,  the  small  orifice  was  sealed  with  a  blow-pipe.  Here, 

Fig.  4G. 


then,  we  have  water  and  its  vapour,  with  scarcely  a  trace 
of  air.  You  hear  how  the  liquid  rings,  exactly  as  it  does  in 
the  case  of  the  water-hammer. 

I  turn  all  the  liquid  into  one  bulb,  A,  which  I  dip  into 
an  empty  glass  to  protect  it  from  currents  of  air.  The 
empty  bulb,  B,  I  plunge  into  a  freezing  mixture  ;  thus,  the 
vapour  which  escapes  from  the  liquid  in  the  bulb,  A,  is  con- 
densed by  the  cold,  to  water,  in  B.  This  condensation 
permits  of  the  formation  of  new  quantities  of  vapour.  As 
the  evaporation  continues,  the  water  which  supplies  the 
vapour  becomes  more  and  more  chilled.  In  a  quarter  of 
an  hour,  or  twenty  minutes,  it  will  be  converted  into  a 
cake  of  ice.  Here  is  the  opalescent  solid  formed  in  a  sec- 
ond instrument,  which  you  saw  me  arranging  before  the 
commencement  of  the  lecture.  The  whole  process  consists 
in  the  uncompensated  transfer  or  motion  from  the  one  bulb 
to  the  other. 


174  LECTURE   V. 

But  the  most  striking  example  of  the  consumption  of 
heat  in  changing  the  state  of  aggregation  is  furnished  by 
the  substance  which  I  have  imprisoned  in  this  strong  iron 
bottle.  This  bottle  contains  carbonic  acid,  liquefied  by 
enormous  pressure.  The  substance  you  know  is  a  gas  under 
ordinary  circumstances ;  here  is  a  jar  full  of  it,  which, 
though  it  manifests  its  nature  by  extinguishing  a  taper,  is 
not  to  be  distinguished,  by  the  eye,  from  common  air. 
When  the  cock  attached  to  the  iron  bottle  is  turned,  the 
pressure  which  acts  upon  the  gas  is  relieved,  the  liquid 
boils — flashes,  as  it  were,  suddenly  into  gas,  which  rushes 
from  the  orifice  with  impetuous  force.  But  you  can  see 
this  current  of  gas  ;  mixed  up  with  it  you  see  a  white  sub- 
stance, which  is  now  blown  against  me,  to  a  distance  of 
eight  or  ten  feet,  through  the  air.  What  is  this  white 
substance  ?  It  is  carbonic  acid  snow.  The  cold  produced 
in  passing  from  the  liquid  to  the  gaseous  state  is  so  intense 
that  a  portion  of  the  carbonic  acid  is  actually  frozen  to  form 
this  snow,  and  mingles  in  small  flakes  with  the  issuing 
stream  of  gas.  I  can  collect  this  snow  in  a  suitable  vessel. 
Here  is  a  cylindrical  box  with  two  hollow  handles,  through 
which  I  will  allow  the  gas  to  pass.  Right  and  left  you  see 
the  streams,  but  a  large  portion  of  the  frozen  mass  is  re- 
tained in  the  box.  I  open  it,  and  you  see  it  filled  with  this 
perfectly  white  carbonic  acid  snow. 

The  solid  very  gradually  disappears ;  its  conversion  into 
vapour  is  slow,  because  it  can  only  slowly  collect  from  sur- 
rounding substances  the  heat  necessary  to  vaporise  it.  You 
can  handle  it  freely,  but  not  press  it  too  much,  lest  it  should 
burn  you.  It  is  cold  enough  to  burn  the  hand.  I  plunge 
a  piece  of  it  into  water,  and  hold  it  there :  you  see  bubbles 
rising  through  the  water — these  are  pure  carbonic  acid  gas. 
I  collect  this  gas,  and  show  you  that  it  possesses  all  the 
properties  of  the  gas  as  commonly  prepared.  The  solid 
acid  does  not  melt  in  the  water  ;  when  I  release  it,  it  rises 


SOLID   CAKBONIC   ACID.  175 

to  the  surface,  and  floats  upon  it.  I  put  a  bit  of  the  acid 
into  my  mouth,  taking  care  not  to  inhale  while  it  is  there. 
I  breathe  against  this  candle  ;  my  breath  extinguishes  the 
flame.  Before  the  conclusion  of  the  lecture,  I  will  show 
you  how  it  is  possible  to  preserve  so  cold  a  body  in  the 
mouth  without  injury.  A  piece  of  iron  of  equal  coldness 
would  do  serious  damage. 

Here,  then,  we  have  a  solid  body  intensely  cold,  which, 
however,  does  not  chill  bodies  in  contact  with  it,  as  it 
might  be  expected  to  do.  In  fact,  no  real  contact  has  been 
established  with  the  acid.  Water,  we  see,  will  not  dissolve 
it,  but  sulphuric  ether  will ;  and  by  pouring  a  quantity  of 
this  ether  on  the  snow,  I  obtain  a"  pasty  mass,  which  has 
an  enormous  power  of  refrigeration.  Here  I  have  some 
thick  and  irregular  masses  of  glass — the  feet,  in  fact,  of 
drinking-glasses.  I  place  a  portion  of  the  solid  acid  on 
them,  and  wet  it  with  ether  ;  you  hear  the  glass  crack  ;  it 
has  been  shattered  by  the  contraction  produced  by  the  in- 
tense cold. 

In  this  basin  I  spread  a  little  paper,  and  over  the  paper 
I  pour  a  pound  or  two  of  mercury ;  on  the  mercury  I  place 
some  solid  carbonic  acid,  and  over  the  acid  I  pour  a  little 
ether.  Mercury,  you  know,  requires  a  very  low  tempera- 
ture to  freeze  it.  Well,  here  it  is  frozen  ;  I  turn  it  out  be- 
fore you,  a  solid  mass  ;  I  can  hammer  the  solid ;  I  can  also 
cut  it  with  a  knife.  To  enable  me.  to  lift  the  mercury  out 
of  the  basin,  I  have  dipped  this  wire  into  it ;  by  this  I 
raise  it,  and  plunge  it  into  a  glass  jar  containing  water.  It 
liquefies,  and  showers  downwards  through  the  water  ;  but 
every  fillet  of  mercury  freezes  the  water  with  which  it 
comes  into  contact,  and  thus  round  each  fillet  is  formed  a 
tube  of  ice,  through  which  you  can  see  the  liquid  metal 
descending.  These  experiments  might  be  multiplied  al- 
most indefinitely  ;  but  enough,  I  trust,  has  been  shown  to 
illustrate  our  present  subject. 


176 


LECTUEE   V. 


Fig.  4T. 


I  have  now  to  direct  your  attention  to  another  and  very 
singular  class  of  phenomena,  connected  with  the  production 
of  vapour.  Here  is  a  broad  porcelain  basin,  B  (fig.  47), 
filled  with  hot  water.  Here  is  a  silver  basin,  s,  which  I 
now  heat  to  redness.  If  I  place  the  silver  basin  in  the  hot 
water,  what  will  occur  ?  You  might  naturally  reply,  that 

the  basin  will  impart  its 
excess  of  heat  instantly  to 
the  water,  and  be  cooled 
down  to  the  temperature 
of  the  latter.  But  nothing 
of  this  kind  occurs.  The 
basin  for  a  time  developes 
a  sufficient  amount  of 
vapour  underneath  it,  to 

lift  it  entirely  out  of  contact  with  the  water ;  or,  in  the  lan- 
guage of  the  hypothesis,  developed  in  our  third  lecture,  it 
is  lifted  by  the  discharge  of  molecular  projectiles  against  its 
under  surface.  This  will  go  on  until  the  temperature  of 
the  basin  sinks,  and  it  is  no  longer  able  to  produce  vapour 
of  sufficient  tension  to  support  it.  Then  it  comes  into  con- 
tact with  the  water,  and  the  ordinary  hissing  of  a  hot 
metal,  together  with  the  cloud  which  forms  overhead,  de- 
clares the  fact. 

I  now  reverse  the  experiment,  and  instead  of  placing 
the  basin  in  the  water,  I  place  the  water  in  the  basin — first 
of  all,  however,  heating  the  latter  to  redness  by  a  lamp. 
You  hear  no  noise  of  ebullition,  no  hissing  of  the  water  as 
I  pour  it  into  the  hot  basin ;  the  drop  rolls  about  on  its 
own  vapour — that  is  to  say,  it  is  sustained  by  the  recoil  of 
the  molecular  projectiles  discharged  from  its  under  surface. 
I  withdraw  the  lamp,  and  allow  the  basin  to  cool,  until  it  is 
no  longer  able  to  produce  vapour  strong  enough  to  support 
the  drop.  The  liquid  then  touches  the  metal ;  the  instant 


SPHEROIDAL    STATE.  177 

it  does  so,  violent  ebullition  sets  in,  and  the  cloud  which 
you  now  observe  forms  above  the  basin. 

You  cannot,  from  your  present  position,  see  this  flat- 
tened spheroid  rolling  about  in  the  hot  basin,  but  I  can 
show  it  to  you,  and,  if  I  am  fortunate,  I  shall  show  you 
something  very  beautiful.  You  will  bear  in  mind  that 
there  is  an  incessant  developement  of  vapour  underneath 
the  drop,  which,  as  incessantly,  escapes  from  it  laterally. 
If  the  drop  rest  upon  a  flattish  surface,  so  that  the  lateral 

Fig.  48. 


escape  is  very  difficult,  the  vapour  will  burst  up  through  the 
middle  of  the  drop.  But  I  have  here  arranged  matters,  so 
that  the  vapour  shall  issue  laterally  ;  and  it  sometimes  hap- 
pens that  the  escape  of  the  vapour  is  rythmic  ;  it  issues  in 
regular  pulses,  and  then  we  have  our  drop  of  water  mould- 
ed to  a  most  beautiful  rosette.  I  have  it  now, — a  round 
mass  of  liquid,  two  inches  in  diameter,  with  a  beautifully 
crimped  border.  I  will  throw  the  beam  of  the  electric 
lamp  upon  this  drop  so  as  to  illuminate  it,  and  holding  this 
lens  over  it,  I  hope  to  cast  its  image  on  the  ceiling,  or  on 
the  screen.  There  it  is  (fig.  48),  a  figure  eighteen  inches  in 
8* 


178  LECTURE   V. 

diameter,  and  the  vapour  breaking,  as  if  in  music,  from  its 
edge.  If  I  add  a  little  ink,  so  as  to  darken  the  liquid,  the 
definition  of  its  outline  is  augmented,  but  the  pearly  lustre 
of  its  surface  is  lost.  I  withdraw  the  heat ;  the  undulation 
continues  for  some  time  :  the  border  finally  becomes  uriin- 
dented.  The  drop  is  now  perfectly  motionless — a  liquid 
spheroid — and  now  it  suddenly  spreads  upon  the  surface, 

Fiar.  49. 


contact  has  been  established,  and  the  spheroidal  condition 
ends. 

I  dry  the  silver  basin  and  place  it,  with  its  bottom  up- 
wards, in  front  of  the  electric  lamp,  and  with  a  lens  in 
front  I  bring  the  rounded  outline  of  the  basin  to  a  focus 
on  the  screen ;  I  dip  this  bit  of  sponge  in  alcohol  and 
squeeze  it  over  the  cold  basin,  so  that  the  drops  fall  upon 
the  surface  of  the  metal :  you  see  their  magnified  images 
upon  the  screen,  and  you  observe  that  wrhen  they  strike  the 
surface  they  spread  out  and  trickle  down  along  it.  Now  I 
will  heat  this  basin  by  placing  a  lamp  underneath.  Ob- 


THE   DROP   IS   SUPPORTED   ON   A   VAPOUR   SPRING.     1Y9 

serve  what  occurs :  when  I  squeeze  the  sponge  the  drops 
descend  as  before,  but  when  they  come  in  contact  with  the 
basin  they  no  longer  spread  but  roll  over  the  surface  as 
liquid  spheres  (fig.  49).  See  how  they  bound  and  dance 
as  if  they  had  fallen  upon  elastic  springs ;  and  so  in  fact 
they  have.  Every  drop,  as  it  strikes  the  hot  surface,  and 
as  it  rolls  along  the  surface,  developes  vapour  which  lifts 
it  out  of  contact,  thus  destroying  all  cohesion  between  the 
surface  and  the  drop,  and  enabling  the  latter  to  preserve  its 
spherical  or  spheroidal  form. 

I  have  here  an  arrangement  suggested  by  Professor 
Poggendorf,  which  shows,  in  a  very  beautiful  manner,  the 
interruption  of  contact  between  the  spheroidal  drop  and  its 
supporting  surface.  From  this  silver  basin,  B  (fig.  50),  in- 

Fig.  50. 


tended  to  hold  the  drop,  I  carry  a  wire,  w9  round  yonder 
magnetic  needle  ;  the  other  end  of  the  galvanometer  wire 
I  attach  to  one  end  of  this  battery,  A.  From  the  opposite 
pole  of  the  little  battery  I  carry  a  wire,  w'^  and  so  attach 
it  to  the  arm,  a  #,  of  this  retort-stand,  #,  that  I  can  readily 
lower  it.  I  heat  the  basin,  pour  in  the  water,  and  lower 
my  wire  till  the  end  of  it  dips  into  the  spheroidal  mass : 
you  see  no  motion  of  the  galvanometer  needle ;  the  only 


180 


LECTUKE   V. 


gap  in  the  entire  circuit  is  that  which  now  exists  under- 
neath the  drop.  If  the  drop  were  in  contact  the  current 
would  pass.  I  prove  this  thus  :  I  withdraw  the  lamp  ;  the 
spheroidal  state  will  soon  end ;  the  liquid  will  touch  the 
bottom.  It  now  does  so,  and  the  needle  instantly  flies 
aside. 

You  can  actually  see  the  interval  between  the  drop  and 
the  hot  surface  upon  which  it  rests.  A  private  experiment 
may  be  made  in  this  way:  Let  a  flattish  basin,  B  (fig.  51), 
be  turned  upside  down,  and  let  the  bottom  of  it  be  slightly 
indented  so  as  to  be  able  to  bear  a  drop  ;  heat  the  basin  by 
a  spirit  lamp,  and  place  upon  it  a  drop  of  ink,  (?,  with  which 
a  little  alcohol  has  been  mixed.  Stretch  a  platinum  wire, 

Fig.  51. 


a  b,  vertically  behind  the  drop,  and  render  the  wire  incan- 
descent by  sending  a  current  of  electricity  through  it.  Bring 
your  eye  to  a  level  with  the  bottom  of  the  drop,  and  you 
will  be  able  to  see  the  red-hot  wire  through  the  interval 
between  the  drop  and  the  surface  which  supports  it.  Let 
me  show  you  this  interval.  I  place  my  basin,  B  (fig.  52), 
as  before,  with  its  bottom'  upward  in  front  of  the  lamp  ;  I 
heat  the  basin  and  bring  carefully  down  upon  it  a  drop,  d, 
dependent  from  a  pipette.  When  it  rests  upon  the  prop- 


INTERVAL  BETWEEN  DKOP  AND  HOT  SURFACE.   181 

er  part  of  the  surface,  and  the  lens  in  front  is  brought  to 
its  proper  position,  you  see  a  line  of  bright  light  between 
the  drop  and  the  silver,  indicating  that  the  beam  of  the 
lamp  has  passed  underneath  the  drop  to  the  screen. 

The  spheroidal  condition  was  first  observed  by  Leiden- 
frost,  and  I  might  give  you  fifty  other  illustrations  of  it. 


Liquids  can  be  made  to  roll  on  liquids.  If,  moreover,  I 
take  this  red-hot  copper  ball  and  plunge  it  into  a  vessel  of 
hot  water,  a  loud  sputtering  is  produced,  due  to  the  escape 
of  the  vapour  generated ;  still  the  contact  of  the  liquid  and 
solid  is  only  very  partial :  let  the  ball  cool,  the  liquid  at 
length  touches  it,  and  then  the  ebullition  is  so  violent  as 
to  project  the  water  from  the  vessel  on  all  sides. 

M.  Boutigny  has  of  late  lent  new  interest  to  this  sub- 
ject by  expanding  the  field  of  illustration,  and  applying  it 
to  the  explanation  of  many  extraordinary  effects.  If  the 
hand  be  wet,  it  may  be  passed  though  a  stream  of  molten 
metal  without  injury.  I  have  seen  M.  Boutigny  myself  pass 
his  wet  hand  through  a  stream  of  molten  iron,  and  toss  with 
his  fingers  the  fused  metal  from  a  crucible :  a  blacksmith 
will  lick  a  white  hot  iron  without  fear  of  burning  his 


182  LECTUEE   V. 

tongue.  The  tongue  is  effectually  preserved  from  contact 
•with  the  iron,  by  the  vapour  developed ;  and  it  was  to  the 
vapour  of  the  carbonic  acid,  which  shielded  me  from  its 
contact,  that  I  owed  my  safety  when  I  put  the  substance 
into  my  mouth.  To  the  same  protective  influence  many 
escapes  from  the  fiery  ordeal  of  ancient  times  have  been 
attributed  by  M.  Boutigny.  I  may  add,  that  the  explana- 
tion of  the  spheroidal  condition  given  by  M.  Boutigny  has 
not  been  accepted  by  scientific  men. 

Boiler  explosions  have  also  been  ascribed  to  the  water 
in  the  boiler  assuming  the  spheroidal  state  ;  the  sudden 
developement  of  steam,  by  subsequent  contact  with  the 
heated  metal,  causing  the  explosion.  We  are  more  igno- 
rant of  these  things  than  we  ought  to  be.  Experimental 

Fig.  53. 


science  has  brought  a  series  of  true  causes  to  light,  which 
may  produce  these  terrible  catastrophes,  but  practical  sci- 
ence has  not  yet  determined  the  extent  to  which  they  ac- 
tually come  into  operation.  The  effect  of  a  sudden  genera- 
tion of  steam  has  been  illustrated  by  an  experiment  which 
I  will  now  make  in  your  presence.  Here  is  a  copper  ves- 
sel, v  (fig.  53),  with  a  neck  which  I  can  stop  with  this 
cork,  through  which  half  an  inch  of  fine  glass  tubing  passes. 


FIEKY   ORDEAL  :   BOILER  EXPLOSIONS.  183 

I  heat  the  copper  vessel,  and  pour  into  it  a  little  water. 
The  liquid  is  now  in  the  spheroidal  state.  I  cork  the  vessel, 
and  the  small  quantity  of  steam  developed,  while  the  water 
remains  spheroidal,  escapes  through  the  glass  tube.  I  now 
remove  the  vessel  from  the  lamp,  and  wait  for  a  minute  or 
two :  very  soon  the  water  will  come  into  contact  with  the 
copper ;  it  now  does  so,  and  you  observe  the  result :  the 
cork  is  driven,  as  if  by  the  explosion  of  gunpowder,  to  a 
considerable  height  in  the  atmosphere. 

I  have  reserved  what  you  will  probably  think  the  most 
interesting  experiment  in  connection  with  this  subject,  for 
the  conclusion  of  to-day's  lecture.     M.  Boutigny,  by  means 
of  sulphurous  acid,  first  froze  water  in  a  red-hot  crucible  ; 
and  Mr.  Faraday  subsequently  froze  mercury,  by  means  of 
solid  carbonic  acid.    I  will  try  and  reproduce  this  latter  re- 
sult ;  but  first  let  me  operate  with  water.     I  have  here  a 
hollow  sphere  of  brass  about  two  inches  in  diameter,  now 
accurately  filled  with  water ;  into  the  sphere  I  have  had 
this  wire  screwed,  which  is  to  serve  as  a  handle.    I  heat 
this  platinum  crucible  to  glowing  redness,  and  place  within 
it  some  lumps  of  solid  carbonic  acid.     I  pour  some  ether  on 
the  acid — neither  of  them  comes  into  contact  with  the  hot 
crucible — they  are  protected  from  contact  by  the  elastic 
cushion  of  vapour  which  surrounds  them ;    I  lower  my 
sphere  of  water  down  upon  the  mass,  and  carefully  pile 
fragments  of  carbonic  acid  over  it,  adding  also  a  little 
ether.     The  pasty  mass  within  the  red-hot  crucible  remains 
intensely  cold  ;  and  now  you  hear  a  crack  !     I  am  thereby 
assured  that  the  experiment  will  succeed.     The  freezing 
water  has  burst  the  brass  sphere,  as  it  burst  the  iron  bottles 
in  a  former  experiment.     Round  the  sphere  I  have  wound 
a  bit  of  wire  to  prevent  the  ice  from  falling  out.     I  now 
raise  the  sphere,  peel  off  the  shattered  brass  shell,  and  there 
you  have  a  solid  sphere  of  ice,  extracted  from  the  red-hot 
crucible. 


184:  .  LECTURE   V. 

I  place  a  quantity  of  mercury  in  a  conical  copper  spoon, 
and  dip  it  into  the  crucible.  The  ether  in  the  crucible  has 
taken  fire,  which  I  did  not  intend  it  to  do.  The  experiment 
ought  to  be  so  made,  that  the  carbonic  acid  gas — the  choke- 
damp  of  mines — ought  to  keep  the  ether  from  ignition. 
But  the  mercury  will  freeze  notwithstanding.  Out  of  the 
fire,  and  through  the  flame,  I  draw  the  spoon,  and  there  is 
the  frozen  mass  turned  out  before  you  on  the  table. 


LECTURE    VI 

[February  27,  1862.] 

CONYECTION  OP  HEATED  AIR — WINDS — THE  UPPER  AND  LOWER  '  TRADES  * 
EFFECT  OP  THE  EARTH'S  ROTATION  ON  THE  DIRECTION  OF  WIND IN- 
FLUENCE OF  AQUEOUS  TAPOUR  UPON  CLIMATE — EUROPE  THE  CONDENSER 
OF  THE  WESTERN  ATLANTIC — RAINFALL  IN  IRELAND — THE  GULP  STREAM 
— FORMATION  OF  SNOW — FORMATION  OP  ICE  FROM  SNOW — GLACIERS — 
PHENOMENA  OF  GLACIER  MOTION — REGELATION — MOULDING  OF  ICE  BY 
PRESSURE — ANCIENT  GLACIERS. 


APPENDIX: — DATA  CONCERNING  GLACIER  MOTION. 

I  PROPOSE  devoting  an  hour  to-day  to  the  considera- 
tion of  some  of  the  physical  phenomena  which  exhibit 
themselves  on  a  large  scale  in  Nature.  And  first,  with  re- 
gard to  winds.  You  see  those  sunburners  now  almost 
wholly  turned  down,  which  are  intended  to  illuminate  this 
room  when  the  daylight  is  intercepted  or  gone.  "Not  to 
give  light  alone  were  they  placed  there  ;  they  were  set  up, 
in  part,  to  promote  ventilation.  The  air,  heated  by  the 
gas  flames,  expands,  and  issues  in  a  strong  vertical  current 
into  the  atmosphere.  The  air  of  the  room  is  thereby  inces- 
santly drawn  upon,  and  a  fresh  supply  must  be  introduced 
to  make  good  the  loss.  Our  chimney  draughts  are  so 
vertical  winds  due  to  the  heating  of  the  air  by  our  fir 

I  ignite  this  piece  of  brown  paper,  the  flame  ascends ;  I 
blow  out  the  flame,  leaving  the  edges  of  the  paper  smok- 
ing ;  the  heated  edges  warm  the  air,  and  produce  currents 
which  carry  the  smoke  upward.  I  dip  the  smoking  paper 


186  LECTURE   VI. 

into  a  large  glass  vessel,  and  stop  the  neck  of  the  vessel  to 
prevent  the  escape  of  the  smoke ;  the  smoke  ascends  with 
the  light  air  in  the  middle,  spreads  out  laterally  above,  is 
cooled,  and  falls  like  a  cascade  of  cloud  along  the  sides  of 
the  vessel.  I  have  here  a  heavy  iron  spatula,  heated  to 
dull  redness  ;  as  I  hold  it  thus,  you  cannot  see  the  currents 
of  heated  air  ascending  from  it.  But  I  can  show  them  to 
you  by  their  action  on  strong  light.'  I  place  the  spatula  in 
the  beam  of  the  electric  lamp ;  here  is  the  shadow  of  the 
spatula  on  the  screen,  and  those  waving  lines  of  light  and 
shade  mark  the  streaming  upwards  of  the  heated  air.  Here 
also  is  an  iron  spoon  containing  a  fragment  of  sulphur, 
wThich  I  heat  until  it  ignites  ;  I  plunge  the  sulphur  into  this 
jar  of  oxygen  :  the  combustion  becomes  more  brilliant  and 
energetic,  and  the  air  of  the  jar  is  thrown  into  intense  com- 
motion. The  fumes  of  the  sulphur  enable  you  to  track  the 
storms  which  the  heating  of  the  air  produces  within  the 
jar.  I  use  the  word  '  storms '  advisedly,  for  the  hurricanes 
which  desolate  the  earth  are  nothing  more  than  large  illus- 
trations of  the  effect  which  we  have  produced  in  this  glass 
jar. 

From  the  heat  of  the  sun  our  winds  are  all  derived. 
We  live  at  the  bottom  of  an  aerial  ocean,  which  is  to  a  re- 
markable degree  permeable  to  the  sun's  rays,  and  is  but 
little  disturbed  by  their  direct  action.  But  those  rays, 
when  they  fall  upon  the  earth,  heat  its  surface  ;  the  air  in 
contact  with  the  surface  shares  its  heat,  is  expanded,  and 
ascends  into  the  upper  regions  of  the  atmosphere.  Where 
the  rays  fall  vertically  on  the  earth,  the  heating  of  the  sur- 
face is  greatest,  that  is  to  say,  between  the  tropics.  Here 
aerial  currents  ascend  and  flow  laterally  north  and  south 
towards  the  poles,  the  heavier  air  of  the  polar  regions 
streaming  in  to  supply  the  place  vacated  by  the  light  and 
warm  air.  Thus  we  have  an  incessant  circulation.  Yes- 
terday I  made  the  following  experiment  in  the  hot  room 


CONVECTION  OF   HEATED  AIE.  187 

of  a  Turkish  bath.  I  opened  wide  the  door,  and  held  a 
lighted  taper  in  the  doorway,  midway  between  top  and 
bottom.  The  flame  rose  straight  from  the  taper.  I  placed 
the  taper  at  the  bottom,  it  was  blown  violently  inwards  ;  I 
placed  it  at  the  top,  it  was  blown  violently  outwards.  Here 
AVC  had  two  currents,  or  winds,  sliding  over  each  other, 
and  moving  in  opposite  directions.  Thus,  also,  as  regards 
our  hemisphere,  we  have  a  current  from  the  equator  setting 
in  towards  the  north  and  flowing  in  the  higher  regions  of 
the  atmosphere,  and  another  flowing  towards  the  equator  in 
the  lower  regions  of  the  atmosphere.  These  are  the  upper 
and  the  lower  Trade  Winds.  ' 

Were  the  earth  motionless,  these  two  currents  would 
run  directly  north  and  south,  but  the  earth  rotates  from 
west  to  east  round  its  axis  once  in  twenty-four  hours.  In 
virtue  of  this  rotation,  an  individual  at  the  equator  is  car- 
ried round  with  a  velocity  of  1,000  miles  an  hour.  You 
have  observed  what  takes  place  when  a  person  incautiously 
steps  out  of  a  carriage  in  motion.  He  is  animated  by  the 
motion  of  the  carriage,  and  when  his  feet  touch  the  earth 
he  is  thrown  forward  in  the  direction  of  the  motion.  This 
is  what  renders  leaping  from  a  railway  carriage,  when  the 
train  is  at  full  speed,  almost  always  fatal.  As  we  with- 
draw from  the  equator,  the  velocity  due  to  the  earth's  ro- 
tation diminishes,  and  becomes  nothing  at  the  poles.  It  is 
proportional  to  the  radius  of  the  parallel  of  latitude,  and 
diminishes  as  these  circles  diminish  in  size.  Imagine,  then, 
an  individual  suddenly  transferred  from  the  equator  to  a 
place  where  the  velocity,  due  to  rotation,  is  only  900  miles 
an  hour  ;  on  touching  the  earth  here  he  would  be  thrown 
forward  in  an  easterly  direction,  with  a  velocity  of  100 
miles  an  hour,  this  being  the  difference  between  the  equa- 
torial velocity  with  which  he  started,  and  the  velocity  of 
the  earth's  surface  in  his  new  locality. 

Similar  considerations  apply  to  the  transfer  of  air  from 


188  LECTURE   VI. 

the  equatorial  to  the  northern  regions,  and  vice  versa.  At 
the  equator  the  air  possesses  the  velocity  of  the  earth's  sur- 
face there,  and  on  quitting  this  position,  it  not  only  has  its 
tendency  northwards  to  obey,  but  also  a  tendency  to  the 
east,  and  it  must  take  a  resultant  direction.  The  farther  it 
goes  north  the  more  is  it  deflected  from  its  original  course ; 
the  more  it  turns  towards  the  east,  the  more  it  becomes 
what  we  should  call  a  westerly  wind.  The  opposite  holds 
good  for  the  current  proeeeding/rom  the  north ;  this  passes 
from  places  of  slow  motion  to  places  of  quick  motion  i  it  is 
met  by  the  earth ;  hence  the  wind  which  started  as  a  north 
wind  becomes  a  north-east  wind,  and  as  it  approaches  the 
equator  it  becomes  more  and  more  easterly. 

It  is  not  by  reasoning  alone  that  we  arrive  at  a  knowl- 
edge of  the  existence  of  the  upper  atmospheric  current, 
though  reasoning  is  sufficient  to  show  that  compensation 
must  take  place  somehow, — that  a  wind  cannot  blow  in  any 
direction  without  an  equal  displacement  of  air  taking  place 
in  the  opposite  direction.  But  clouds  are  sometimes  seen  in 
the  tropics  high  in  the  atmosphere,  and  moving  in  a  direction 
opposed  to  that  of  the  constant  wind  below.  Could  we  dis- 
charge a  light  body  with  sufficient  force  to  cause  it  to  pen- 
etrate the  lower  current,  and  reach  the  higher,  the  direction 
of  that  body's  motion  would  give  us  the  direction  of  the 
wind  above.  Human  strength  cannot  perform  this  experi- 
ment, but  it  has  nevertheless  been  made.  Ashes  have  been 
shot  through  the  lower  current  by  volcanoes,  and,  from  the 
places  where  they  have  subsequently  fallen,  the  direction 
of  the  wind  which  carried  them  has  been  inferred.  Pro- 
fessor Dove  in  his  '"Witterungs  Verhaltnisse  von  Berlin' 
cited  the  following  instance  :  '  On  the  night  of  April  30th, 
explosions  like  those  of  heavy  artillery  were  heard  at  Barba- 
does,  so  that  the  garrison  at  Fort  St.  Anne  remained  all  night 
under  arms.  On  May  1,  at  daybreak,  the  eastern  portion 
of  the  horizon  appeared  clear,  while  the  rest  of  the  firma- 


CUEEENTS   OF  THE  ATMOSPHEEE.  189 

ment  was  covered  by  a  black  cloud,  which  soon  extended 
to  the  east,  quenched  the  light  there,  and  at  length  pro- 
duced a  darkness  so  dense  that  the  windows  in  the  rooms 
could  not  be  discerned.  A  shower  of  ashes  descended, 
under  which  the  tree  branches  bent  and  broke.  Whence 
came  these  ashes  ?  From  the  direction  of  the  wind,  we 
should  infer  that  they  came  from  the  Peak  of  the  Azores : 
they  came,  however,  from  the  volcano  Morne  Garou  in  St. 
Vincent,  which  lies  about  100  miles  west  of  Barbadoes.  The 
ashes  had  been  cast  into  the  current  of  the  upper  trade.  A 
second  example  of  the  same  kind  occurred  on  January  20, 
1835.  On  the  24th  and  25th  the  sun  was  darkened  in  Ja- 
maica by  a  shower  of  fine  ashes,  which  had  been  discharged 
from  the  mountain  Coseguina,  distant  800  miles.  The  peo- 
ple learned  in  this  way  .that  the  explosions  previously  heard 
were  not  those  of  artillery.  These  ashes  could  only  have 
been  carried  by  the  upper  current,  as  Jamaica  lies  north- 
east from  the  mountain.  The  same  eruption  gives  also  a 
beautiful  proof  that  the  ascending  air-current  divides  itself 
above,  for  ashes  fell  upon  the  ship  Conway  in  the  Pacific, 
at  a  distance  of  700  miles  south-west  of  Coseguina. 

'  Even  on  the  highest  summits  of  the  Andes  no  traveller 
has  as  yet  reached  the  upper  trade.  From  this  some  notion 
may  be  formed  of  the  force  of  the  explosions  ;  they  were 
indeed  tremendous  in  both  instances.  The  roaring  of  Cose- 
guina was  heard  at  San  Salvador,  a  distance  of  1,600  miles. 
Union,  a  seaport  on  the  west  coast  of  Conchagua,  was  in 
absolute  darkness  for  forty-three  hours ;  as  light  began  to 
dawn  it  was  observed  that  the  sea-shore  had  advanced  800 
feet  upon  the  ocean,  through  the  mass  of  ashes  which  had 
fallen.  The  eruption  of  Morne  Garou  forms  the  last  link 
of  a  chain  of  vast  volcanic  actions.  In  June  and  July 
1811,  near  St.  Miguel,  one  of  the  Azores,  the  island  Sabri- 
na  rose,  accompanied  by  smoke  and  flame,  from  the  bottom 
of  a  sea  150  feet  deep,  attained  a  height  of  300  feet  and  a 


190  LECTURE   VI. 

circumference  of  a  mile.  The  small  Antilles  were  after- 
wards shaken,  and  subsequently  the  valleys  of  the  Missis- 
sippi, Arkansas,  and  Ohio.  But  the  elastic  forces  found  no 
vent ;  they  sought  one,  then,  on  the  north  coast  of  Colum- 
bia. March  26  began  as  a  day  of  extraordinary  heat  in 
Caraccas ;  the  air  was  clear  and  the  firmament  cloudless. 
It  was  Green  Thursday,  and  a  regiment  of  troops  of  the 
line  stood  under  arms  in  the  barracks  of  the  quarter  San 
'Carlos  ready  to  join  in  the  procession.  The  people 
streamed  to  the  churches.  A  loud  subterranean  thunder 
was  heard,  and  immediately  afterwards  followed  an  earth- 
quake shock  so  violent,  that  the  church  of  Alta  Gracia,  150 
feet  in  height,  borne  by  pillars  fifteen  feet  thick,  formed  a 
heap  of  crushed  rubbish  not  more  than  six  feet  high.  In 
the  evening  the  almost  full  moon  looked  down  with  mild 
lustre  upon  the  ruins  of  the  town,  under  which  lay  the 
crushed  bodies  of  upwards  of  10,000  of  its  inhabitants. 
But  even  here  there  was  no  exit  granted  to  the  elastic 
forces  underneath.  Finally,  on  April  27,  they  succeeded  in 
opening  once  more  the  crater  of  Morne  Garou,  which  had 
been  closed  for  a  century ;  and  the  earth,  for  a  distance 
equal  to  that  from  Vesuvius  to  Paris,  rung  with  the  thun- 
der-shout of  the  liberated  prisoner.' 

I  have  here  a  terrestrial  globe,  on  which  I  now  trace 
with  my  hand  two  meridians  ;  they  start  from  the  equator 
of  the  globe  a  foot  apart,  which  would  correspond  to  about 
1,000  miles  on  the  earth's  surface.  But  these  meridians, 
as  they  proceed  northward,  gradually  approach  each  other, 
and  meet  at  the  north  pole.  It  is  manifest  that  the  air 
which  rises  between  these  meridians  in  the  equatorial  re- 
gions must,  if  it  went  direct  to  the  pole,  squeeze  itself  into 
an  ever-narrowing  bed.  Were  the  earth  a  cylinder  instead 
of  a  sphere,  we  might  have  a  circulation  from  the  middle 
of  the  cylinder  quite  to  each  end,  and  a  return  current  from 
each  end  to  the  middle.  But  this,  in  the  case  of  the  earth. 


THE  TJPPEK   AND  LOWER  TEADES.  191 

is  impossible,  simply  because  the  space  around  the  poles  is 
unable  to  embrace  the  air  from  the  equator.  The  cooled 
equatorial  air  sinks,  and  the  return  current  sets  in  before 
the  poles  are  attained,  and  this  occurs  more  or  less  irregu- 
larly. The  two  currents,  moreover,  instead  of  flowing  one 
over  the  other,  often  flow  beside  each  other.  They  con- 
stitute rivers  of  air,  with  incessantly  shifting  beds. 

These  are  the  great  winds  of  our  atmosphere  which, 
however,  are  materially  modified  by  the  irregular  distribu- 
tion of  land  and  water.  Winds  of  minor  importance  also 
occur,  through  the  local  action  of  heat,  cold,  and  evapora- 
tion. There  are  winds  produced  by  the  heating  of  the  air 
in  Alpine  valleys,  and  which  sometimes  rush  with  sudden 
and  destructive  violence  down  the  gulleys  of  the  moun- 
tains :  gentler  down-flows  of  air  are  produced  by  the  pres- 
ence of  glaciers  upon  the  heights.  There  are  the  land 
breeze  and  the  sea  breeze,  due  to  the  varying  temperature 
of  the  sea-board  soil,  by  day  and  night.  The  morning  sun 
heating  the  land,  produces  vertical  displacement,  and  the 
air  from  the  sea  moves  landward.  In  the  evening  the  land 
is  more  chilled  by  radiation  than  the  sea,  and  the  conditions 
are  reversed  ;  the  heavy  air  of  the  land  now  flows  seaward. 

Thus,  then,  a  portion  of  the  heat  of  the  tropics  is  sent 
by  an  aerial  messenger  towards  the  poles,  a  more  equable 
distribution  of  terrestrial  warmth  being  thus  secured.  But 
in  its  flight  northward  the  air  is  accompanied  by  another 
substance — by  the  vapour  of  water,  which,  you  know,  is 
perfectly  transparent.  Imagine  the  ocean  of  the  tropics, 
giving  forth  its  vapour,  which  promotes  by  its  lightness  the 
ascent  of  the  associated  air.  They  expand  as  they  ascend  : 
at  a  height  of  16,000  feet  the  air  and  vapour  occupy  twice 
the  volume  which  they  embraced  at  the  sea  level.  To  se- 
cure this  space  they  must,  by  their  elastic  force,  push  away 
the  air  in  all  -directions  round  them ;  they  must  perform 
work ;  and  this  work  cannot  be  performed,  save  at  the  ex- 


192  LECTUKE   VI. 

pense  of  the  warmth  with  which  they  were  in  the  first  in- 
stance charged. 

The  vapour  thus  chilled  is  no  longer  competent  to  retain 
the  gaseous  form.  It  is  precipitated  as  cloud :  the  cloud 
descends  as  rain ;  and  in  the  region  of  calms,  or  directly 
under  the  sun,  where  the  air  is  first  drained  of  its  aqueous 
load,  the  descent  of  rain  is  enormous.  The  sun  does  not 
remain  always  vertically  over  the  same  parallel  of  latitude 
— he  is  sometimes  north  of  the  equator,  sometimes  south 
of  the  equator,  the  two  tropics  limiting  his  excursion. 
When  he  is  south  of  the  equator,  the  earth's  surface  north 
of  it  is  no  longer  in  the  region  of  calms,  but  in  a  region 
across  which  the  aerial  current  from  the  north  flows 
towards  the  region  of  calms.  The  moving  air  is  but 
slightly  charged  with  vapour,  and  as  it  travels  from  north 
to  south  it  becomes  ever  warmer ;  it  constitutes  a  dry 
wind,  and  its  capacity  to  retain  vapour  is  continually  aug- 
menting. It  is  plain,  from  these  considerations,  that  each 
place  between  the  tropics  must  have  its  dry  season  and 
rainy  season ;  dry  when  the  sun  is  at  the  opposite  side  of 
the  equator,  and  wet  when  the  sun  is  overhead. 

Gradually,  however,  as  the  upper  stream,  which  rises 
from  the  equator,  and  flows  towards,  the  poles,  becomes 
chilled  and  dense,  it  sinks  towards  the  earth  ;  at  the  Peak 
of  Tenerifie  it  has  already  sunk  below  the  summit  of  the 
mountain.  With  the  contrary  wind  blowing  at  the  base, 
the  traveller  finds  the  stream  from  the  equator  blowing 
strong  over  the  top.  Farther  north  the  equatorial  wind 
sinks  lower  still,  and  finally  quite  reaches  the  surface  of 
the  earth.  Europe,  for  the  most  part,  is  overflowed  by 
this  equatorial  current.  Here  in  London,  for  eight  or  nine 
months  in  the  year,  south-westerly  winds  prevail.  But 
mark  what  an  influence  this  must  have  upon  our  climate. 
The  moisture  of  the  equatorial  ocean  comes  to  us  endowed 
with  potential  energy ;  with  its  molecules  separate,  and 


ATMOSPHEEIC   CONDENSATION.  193 

therefore  competent  to  clash  and  develope  heat  by  their 
collision ;  it  comes,  if  you  will,  charged  with  latent  heat. 
In  our  northern  atmosphere  the  collision  takes  place,  and 
the  heat  generated  is  a  main  source  of  warmth  to  our  cli- 
mate. Were  it  not  for  the  rotation  of  the  earth,  we  should 
have  over  us  the  hot  dry  blasts  of  Africa ;  but  owing  to 
this  rotation,  the  wind  which  starts  northward  from  the 
Gulf  of  Mexico  is  deflected  to  Europe.  Europe  is,  there- 
fore, the  recipient  of  those  stores  of  latent  heat  which  were 
amassed  in  the  western  Atlantic.  The  British  Isles  come 
in  for  the  greatest  share  of  this  moisture  and  heat,  and  this 
circumstance  adds  itself  to  that  already  dwelt  upon — the 
high  specific  heat  of  water — to  preserve  our  climate  from 
extremes.  It  is  this  condition  of  things  which  makes  our 
fields  so  green,  and  which  gives  the  blossom  to  our  maid- 
ens' cheeks.  A  German  writer,  Moritz,  expresses  himself 
on  these  points  in  the  following  ardent  words : — '  Ye 
blooming  youthful  faces,  ye  green  meadows  and  streams 
of  this  happy  land,  how  have  ye  enchanted  me  !  O  Rich- 
mond, Richmond !  never  can  I  forget  the  evening  when, 
full  of  delight,  I  wandered  near  you  up  and  down  along 
the  flowery  banks  of  the  Thames.  This,  however,  must 
not  detain  me  from  that  dry  and  sand-strewn  soil  on  which 
fate  has  appointed  me  my  sphere  of  action.'  All  this  poe- 
try and  enchantment  are  derived  directly  from  aqueous 
vapour.* 

As  we  travel  eastward  in  Europe,  the  amount  of  aque- 
ous precipitation  grows  less  and  less ;  the  air  becomes  more 
and  more  drained  of  its  moisture.  Even  between  the  east 
and  west  coasts  of  our  own  islands  the  difference  is  sensi- 
ble, and  local  circumstances  also  have  a  powerful  influence 
on  the  amount  of  precipitation.  Dr.  Lloyd  finds  the  mean 
yearly  temperature  of  the  western  coast  of  Ireland  about 

*  Its  relation  to  Radiant  Heat  is  developed  in  Lecture  XI. 
9 


LECTURE  VI. 


two  degrees  higher  than  that  of  the  eastern  coast,  at  the 
same  height,  and  in  the  same  parallel  of  latitude.  The 
total  amount  of  rain  which  fell  in  the  year  1851,  at  various 
stations  in  the  island,  is  given  in  the  following  table  — 


Station 

Rain  in  inches 

Portarlington 

.  21-2 

Killough     . 

.  23-2 

Dublin 

.  26-4 

Athy  .... 

.  26*7 

Donaghadce 

.  27-9 

Courtown    . 

.  29-6 

Kilrush       . 

.  32-6 

.  33-1 

Killybegs    . 

.  33-2 

Dumnore     . 

.  33-5 

Portrush 

.  37.2 

Burincrana 

.  39-3 

Markrce 

.  40-3 

Castletownsend  . 

.  42-5 

Westport    . 

.  45-9 

Cahircivecn 

.  59-4 

With  reference  to  this  table,  Dr.  Lloyd  remarks — 

'  1.  That  there  is  great  diversity  in  the  yearly  amount 
of  rain  at  the  different  stations,  all  of  which  (excepting 
four)  are  but  a  few  feet  above  the  sea  level ;  the  greatest 
rain  (at  Cahirciveen)  being  nearly  three  times  as  great  as 
the  least  (at  Portarlington). 

'  2.  That  the  stations  of  least  rain  are  either  inland  or 
on  the  eastern  coast,  while  those  of  the  greatest  rains  are 
at  or  near  the  western  coast. 

4  3.  That  the  amount  of  ram  is  greatly  dependent  on 
the  proximity  of  a  mountain  chain  or  group,  being  always 
considerable  in  such  neighbourhood,  unless  the  station  lie 
to  the  north-east  of  the  same. 

'  Thus,  Portarlington  lies  to  the  north-east  of  Slieve- 
bloom ;  Killough  to  the  north-east  of  the  Mourne  range ; 
Dublin,  north-east  of  the  Wicklow  range,  and  so  on.  On 


KAIN-FALL   IK   IRELAND.  195 

the  other  hand,  the  stations  of  greatest  rain,  Cahirciveen, 
Castletownsend,  Westport,  &c.,  are  in  the  vicinity  of  high 
mountains,  but  on  a  different  side.'  * 

This  distribution  of  heat  by  the  transfer  of  masses  of 
heated  air  from  place  to  place,  has  been  called '  convection? 
in  contradistinction  to  the  process  of  conduction,  which 
will  be  treated  in  our  next  lecture.  Heat  is  distributed  in 
a  similar  manner  through  liquids.  I  have  here  a  glass  cell, 
c  (fig.  54),  containing  warm  water ;  I  place  it  in  front  of 

Fig.  54. 


the  electric  lamp,  and  by  means  of  a  converging  lens, 
throw  a  magnified  image  of  the  cell  upon  the  screen.  I 
now  introduce  the  end  of  this  pipette  into  the  water  of  the 
cell,  and  allow  a  little  cold  water  to  gently  enter  the  hot. 
The  difference  of  refraction  between  both  enables  you  to 
see  the  heavy  cold  water  falling  through  the  lighter  warm 
water.  The  experiment  succeeds  still  better  when  I  allow 
a  fragment  of  ice  to  float  upon  the  surface  of  the  water. 
As  the  ice  melts,  it  sends  long  heavy  stria3  downwards  to 
the  bottom  of  the  cell.  You  observe,  as  I  cause  the  ice  to 

*  The  greatest  rainfall  recorded  by  Sir  John  Herschel  in  his  table 
(Meteorology,  110,  &c.)  occurs  at  Cherra  Pungee,  where  the  annual  fall  is 
692  inches.  It  is  not  my  object  to  enter  far  into  the  subject  of  meteor- 
ology ;  for  the  fullest  and  most  accurate  information  the  reader  will  refer  to 
the  excellent  works  of  Sir  John  Herschel  and  Professor  Dove. 


196  LECTUKE   VI. 

move  along  the  top,  how  "these  streams  of  cold  water 
descend  through  the  hot.  I  now  reverse  the  experiment, 
placing  cold  water  in  the  cell,  and  hot  water  in  the  pipette. 
Care  is  here  necessary  to  allow  the  warm  water  to  enter 
without  any  momentum,  which  would  carry  it  mechanically 
down.  You  notice  the  effect.  The  point  of  the  pipette  is 
in  the  middle  of  the  cell,  and  you  see,  as  the  warm  water 

Fig.  55. 


enters,  it  speedily  turns  upwards  (fig.  55)  and  overflows  the 
top,  almost  as  oil  would  do  under  the  same  circumstances. 

When  a  vessel  containing  water  is  heated  at  the  bot- 
tom, the  warmth  communicated  is  thus  diffused.  You  may 
see  the  direction  of  the  ascending  warm  currents  by  means 
of  the  electric  lamp,  and  also  that  of  the  currents  which 
descend  to  occupy  the  place  of  the  lighter  water.  Here  is 
a  vessel  containing  cochineal,  the  fragments  of  which, 
being  not  much  heavier  than  the  water,  freely  follow  the 
direction  of  its  currents.  You  see  the  pieces  of  cochineal 
breaking  loose  from  the  heated  bottom ;  ascending  along 
the  middle  of  the  jar,  and  descending  again  by  the  sides. 
In  the  Geyser  of  Iceland  this  convection  occurs  on  a  grand 
scale.  A  fragment  of  paper  thrown  upon  the  centre  of  the 
water  which  fills  the  pipe  is  instantly  drawn  towards  the 
side,  and  there  sucked  down  by  the  descending  current. 

Partly  to  this  cause,  but  mainly,  perhaps,  to  the  action 
of  winds,  currents  establish  themselves  in  the  ocean,  and 


CONVECTION   IN  LIQUIDS.  197 

powerfully  influence  climate  by  the  heat  which  they  dis- 
tribute. The  most  remarkable  of  these  currents,  and  by 
far  the  most  important  for  us,  is  the  so-called  Gulf-stream, 
which  sweeps  across  the  Atlantic  from  the  equatorial  re- 
gions through  the  Gulf  of  Mexico,  whence  it  derives  its 
name.  As  it  quits  the  straits  of  Florida  it  has  a  tempera- 
ture of  83°  Falir.,  thence  it  follows  the  coast  of  America  as 
far  as  Cape  Fear,  whence  it  starts  across  the  Atlantic,  tak- 
ing a  north-easterly  course,  and  finally  washing  the  coast 
of  Ireland,  and  the  north-western  shores  of  Europe  gen- 
erally. As  might  be  expected,  the  influence  of  this  body 
of  warm  water  makes  itself  most  evident  in  our  winter. 
It  then  entirely  abolishes  the  difference  of  temperature 
due  to  the  difference  of  latitude  of  north  and  south  Brit- 
ain ;  if  we  walk  from  the  Channel  to  the  Shetland  Isles,  in 
January,  we  encounter  everywhere  the  same  temperature. 
The  Isothermal  line  runs  north  and  south.  The  presence 
of  the  water  renders  the  climate  of  western  Europe  totally 
different  from  that  of  the  opposite  coast  of  America.  The 
river  Hudson,  for  example,  in  the  latitude  of  Rome,  is 
frozen  over  for  three  months  in  the  year.  Starting  from 
Boston  in  January,  and  proceeding  round  St.  John's,  and 
thence  to  Iceland,  we  meet  everywhere  the  same  tempera- 
ture. The  harbour  of  Hammerfest  derives  great  value  from 
the  fact  that  it  is  clear  of  ice  all  the  year  round.  This  is 
due  to  the  Gulf-stream  which  sweeps  round  the  North 
Cape,  and  so  modifies  the  climate  there,  that  at  some 
places,  by  proceeding  northward,  you  enter  a  warmer  re- 
gion. The  contrast  between  northern  Europe  and  the  east 
coast  of  America  caused  Halley  to  surmise  that  the  north 
pole  of  the  earth  had  shifted ;  that  it  was  formerly  situate 
somewhere  near  Behring's  Straits,  and  that  the  intense 
cold  observed  in  these  regions  is  really  the  cold  of  the  an- 
cient pole,  which  had  not  been  entirely  subdued  since  the 
axis  changed  its  direction.  But  now  we  know  that  the 


108  LECTUKE    VI. 

Gulf-stream  and  the  diffusion  of  heat  by  winds  and  vapours 
are  the  real  causes  of  European  mildness.  On  the  western 
coast  of  America,  between  the  Rocky  mountains  and  the 
ocean,  we  find  a  European  climate. 

Europe,  then,  is  the  condenser  of  the  Atlantic ;  and  the 
mountains  are  the  chief  condensers  in  Europe.  On  them, 
moreover,  when  they  are  sufficiently  high,  the  condensed 
vapour  descends,  not  in  a  liquid,  but  a  solid  form.  Let  us 
look  to  this  water  in  its  birthplace,  and  follow  it  through 
its  subsequent  course.  Clouds  float  in  the  air,  and  hence 
the  surmise  that  they  are  composed  of  vesicles  or  bladders 
of  water,  thus  forming  shells  instead  of  spheres.  Eminent 
travellers  say  that  they  have  seen  these  bubbles,  and  their 
statements  are  entitled  to  all  respect.  It  is  certain,  how- 
ever, that  the  water-particles  at  high  elevations  possess,  on 
or  after  precipitation,  the  powers  of  building  themselves 
into  crystalline  forms  ;  they  thus  bring  forces  into  play 
which  we  have  hitherto  been  accustomed  to  regard  as 
molecular,  and  which  could  not  be  ascribed  to  the  aggre- 
gates necessary  to  form  vesicles. 

Snow,  perfectly  formed,  is  not  an  irregular  aggregate 
of  ice-particles  ;  in  a  calm  atmosphere,  the  aqueous  atoms 
arrange  themselves  so  as  to  form  the  most  exquisite  figures. 
You  have  seen  those  six-petalled  flowers  which  form  them- 
selves within  a  block  of  ice  when  a  beam  of  heat  is  sent 
through  it.  The  snow-crystals,  formed  in  a  calm  atmos- 
phere, are  built  upon  the  same  type  :  the  molecules  arrange 
themselves  to  form  hexagonal  stars.  From  a  central  nuc- 
leus shoot  six  spicula?,  every  two  of  which  are  separated 
by  an  angle  of  60°.  From  these  central  ribs  smaller  spic- 
ulse  shoot  right  and  left  with  unerring  fidelity  to  the 
angle  60°,  and  from  these  again  other  smaller  ones  diverge 
at  the  same  angle.  The  six-leaved  blossoms  assume  the 
most  wonderful  variety  of  form ;  their  tracery  is  of  the 
finest  frozen  gauze ;  and  round  about  their  corners  other 


CAUSE   OF   EUROPEAN"  MILDNESS.  199 

rosettes  of  smaller  dimensions  often  cling.  Beauty  is  su- 
perposed upon  beauty,  as  if  Nature,  once  committed  to  her 
task,  took  delight  in  showing,  even  within  the  narrowest 
limits,  the  wealth  of  her  resources.* 

These  frozen  blossoms  constitute  our  mountain  snows  ; 
they  load  the  Alpine  heights,  where  their  frail  architecture 
is  soon  destroyed  by  the  accidents  of  the  weather.  Every 
winter  they  fall,  and  every  summer  they  disappear,  but  this 
rythmic  action  does  not  perfectly  compensate  itself.  Be- 
low a  certain  line  warmth  is  predominant,  and  the  quantity 
which  falls  every  winter  is  entirely  swept  away ;  above 
this  line  cold  is  predominant,  the  quantity  which  falls  is  in 
excess  of  the  quantity  melted,  and  an  annual  residue  re- 
mains. In  winter  the  snows  reach  to  the  plains ;  in  sum- 
mer they  retreat  to  the  snow-line, — to  that  particular  line 
where  the  snow-fall  of  every  year  is  exactly  balanced  by 
the  consumption,  and  above  which  is  the  region  of  eternal 
snows.  But  if  a  residue  remains  annually  above  the  snow 
line,  the  mountains  must  be  loaded  with  a  burden  which 
increases  every  year.  Supposing  at  a  particular  point 
above  the  line  referred  to,  a  layer  of  three  feet  a  year  is 
added  to  the  mass ;  this  deposit,  accumulating  even  through 
the  brief  period  of  the  Christian  era,  would  produce  an 
elevation  of  5,580  feet.  And  did  such  accumulations  con- 
tinue throughout  geologic  instead  of  historic  ages,  there  is 
no  knowing  the  height  to  which  the  snows  would  pile 
themselves.  It  is  manifest  no  accumulation  of  this  kind 
takes  place  ;  the  quantity  of  snow  on  the  mountains  is  not 
augmenting  in  this  way  ;  for  some  reason  or  other  the  sun 
is  not  permitted  to  lift  the  ocean  out  of  its  basins  and  pile 
its  waters  permanently  upon  the  hills. 

But  how  is  this  annually  augmenting  load  taken  off  the 

*  See  fig.  56,  in  which  are  copied  some  of  the  beautiful  drawings  of 
Mr.  Glaisher. 


200 


SNOW    CEYSTALS.  201 

shoulders  of  the  mountains  ?  The  snows  sometimes  detach 
themselves  and  rush  down  the  slopes  in  avalanches,  melting 
to  w^ater  in  the  warmer  air  below.  But  the  violent  rush 
of  the  avalanche  is  not  their  only  motion ;  they  also  creep 
by  almost  insensible  degrees  down  the  slopes.  As  layer, 
moreover,  heaps  itself  upon  layer,  the  deeper  portions  of 
the  mass  become  squeezed  and  consolidated ;  the  air  first 
entrapped  in  the  meshes  of  the  snow  is  squeezed  out,  and 
the  compressed  mass  approximates  more  and  more  to  the 
character  of  ice.  You  know  how  the  granules  of  a  snow- 
ball will  adhere  ;  you  know  how  hard  you  can  make  it  if 
mischievously  inclined  :  the  snow -ball  is  incipient  ice  ;  aug- 
ment your  pressure,  and  you  actually  convert  it  into  ice. 
But  even  after  it  has  attained  a  compactness  which  would 
entitle  it  to  be  called  ice,  it  is  still  capable  of  yielding 
more  or  less,  as  the  snow  yields,  to  pressure.  When,  there- 
fore, a  sufficient  depth  of  the  substance  collects  upon  the 
earth's  surface,  the  lower  portions  are  squeezed  out  by  the 
pressure  of  the  upper  ones,  and  if  the  snow  rests  upon  a 
slope,  it  will  yield  principally  in  the  direction  of  the  slope, 
and  move  downwards. 

This  motion  is  incessantly  going  on  along  the  slopes  of 
every  snow-laden  mountain;  in  the  Himalayas,  in  the 
Andes,  in  the  Alps  ;  but  in  addition  to  this  motion,  which 
depends  upon  the  power  of  the  substance  itself  to  yield  to 
pressure,  there  is  also  a  sliding  motion  over  the  inclined 
bed.  The  consolidated  snow  moves  bodily  over  the  moun- 
tain slope,  grinding  off  the  asperities  of  the  rocks,  and  pol- 
ishing their  hard  surfaces.  The  under  surface  of  the 
mighty  polisher  is  also  scarred  and  furrowed  by  the  rocks 
over  which  it  has  passed ;  but  as  the  compacted  snow  de- 
scends, it  enters  a  warmer  region,  is  more  copiously  melted 
and  sometimes,  before  the  base  of  its  slope  is  reached,  it  is 
wholly  cut  off  by  fusion.  Sometimes,  however,  large  and 
deep  valleys  receive  the  gelid  masses  thus  sent  down ;  in 
9* 


202  LECTUKE   VI. 

these  valleys  it  is  further  consolidated,  and  through  them 
it  moves,  at  a  slow  but  measurable  pace,  imitating  in  all  its 
motions  those  of  a  river.  The  ice  is  thus  carried  far  be- 
yond the  limits  of  perpetual  snow,  until,  at  length,  the  con- 
sumption below  equals  the  supply  above,  and  at  this  point 
the  glacier  ceases.  From  the  snow-line  downwards  in  sum- 
mer, we  have  ice  j  above  the  snow-line,  both  summer  and 
winter,  we  have,  on  the  surface,  snow.  The  portion  below 
the  snow-line  is  called  a  glacier,  that  above  the  snow-line  is 
called  the  neve.  The  neve,  then,  is  the  feeder  of  the 
glacier. 

Several  valleys  thus  filled  may  unite  in  a  single  valley, 
the  tributary  glaciers  welding  themselves  together  to  form 
a  trunk  glacier.  Both  the  main  valley  and  its  tributaries 
are  often  sinuous,  and  the  tributaries  must  change  their  di- 
rection to  form  the  trunk.  The  width  of  the  valley,  also, 
often  changes ;  the  glacier  is  forced  through  narrow 
gorges,  widening  after  it  has  passed  them ;  the  centre  of 
the  glacier  moves  more  quickly  than  the  sides,  and  the 
surface  more  quickly  than  the  bottom.  The  point  of 
swiftest  motion  follows  the  same  law  as  that  observed  in 
the  flow  of  rivers,  changing  from  one  side  of  the  centre  to 
the  other,  as  the  flexure  of  the  valley  changes.*  Most  of 
the  great  glaciers  in  the  Alps  have,  in  summer,  a  central 
velocity  of  two  feet  a  day.  There  are  points  on  the  Mer- 
de-Glace,  opposite  the  Mont  en  vert,  which  have  a  daily  mo- 
tion of  thirty  inches  in  summer,  and  in  winter  have  been 
found  to  move  at  half  this  rate. 

The  power  of  accommodating  itself  to  the  channel 
through  which  it  moves  has  led  eminent  men  to  assume 
that  ice  is  viscous ;  and  the  phenomena  at  first  sight  seem 
to  enforce  this  assumption.  The  glacier  widens,  bends,  and 
narrows,  and  its  centre  moves  more  quickly  than  its  sides ; 

*  For  the  data  on  which  this  law  is  founded  see  Appendix  to  this  Lecture. 


GLACIERS.  203 

a  viscous  mass  would  undoubtedly  do  the  same.  But  the 
most  delicate  experiments  on  the  capacity  of  ice  to  yield 
to  strain,  to  stretch  out  like  treacle,  honey  or  tar,  have 
failed  to  detect  this  stretching  power.  Is  there,  then,  any 
other  physical  quality  to  which  the  power  of  accommoda- 
tion possessed  by  glacier  ice,  may  be  referred  ? 

Let  us  approach  this  subject  gradually.  We  know  that 
vapour  is  continually  escaping  from  the  free  surface  of  a 
liquid ;  that  the  particles  at  the  surface  attain  their  gaseous 
liberty  sooner  than  the  particles  within  the  liquid ;  it  is 
natural  to  expect  a  similar  state  of  things  with  regard  to 
ice ;  that  when  the  temperature  of  a  mass  of  ice  is  uni' 
formly  augmented,  the  first  particles  to  attain  liquid  liberty 
are  those  at  the  surface ;  for  here  they  are  entirely  free, 
on  one  side,  from  the  controlling  action  of  the  surrounding 
particles.  Supposing,  then,  two  pieces  of  ice  raised 
throughout  to  32°,  and  melting  at  this  temperature  at  their 
surfaces  ;  what  may  be  expected  to  take  place  if  we  place 
the  liquefying  surfaces  close  together?  We  thereby  vir- 
tually transfer  these  surfaces  to  the  centre  of  the  ice,  where 
the  motion  of  each  molecule  is  controlled  all  round  by  its 
neighbours.  As  might  reasonably  be  expected,  the  liberty 
of  liquidity  at  each  point  where  the  surfaces  touch  each 
other,  is  arrested,  and  the  two  pieces  freeze  together  at 
these  points.  Let  us  make  the  experiment :  Here  are  two 
masses  which  I  have  just  cut  asunder  by  a  saw ;  I  place 
their  flat  surfaces  together;  half  a  minute's  contact  will 
suffice  ;  they  are  now  frozen  together,  and  by  taking  hold 
of  one  of  them  I  thus  lift  them  both. 

This  is  the  eifect  to  which  attention  was  first  directed 
by  Mr.  Faraday  in  June  1850,  and  which  is  now  known 
under  the  name  of  Regelation.  On  a  hot  summer's  day,  I 
have  gone  into  a  shop  in  the  Strand  where  fragments  of  ice 
were  exposed  in  a  basin  in  the  window ;  and  with  the 
shopman's  permission  have  laid  hold  of  the  topmost  piece 


204  LECTURE   VI. 

of  ice,  and  by  means  of  it  have  lifted  the  whole  of  the 
pieces  bodily  out  of  the  dish.  Though  the  thermometer  at 
the  time  stood  at  80°,  the  pieces  of  ice  had  frozen  together 
art  their  points  of  junction.  Even  under  hot  water  this 
effect  takes  place ;  I  have  here  a  basin  of  water  as  hot  as 
my  hand  can  bear ;  I  plunge  into  it  these  two  pieces  of 
ice,  and  hold  them  together  for  a  moment :  they  are  now 
frozen  together,  notwithstanding  the  presence  of  the  heat- 
ed liquid.  A  pretty  experiment  of  Mr.  Faraday's  is  to 
place  a  number  of  small  fragments  of  ice-  in  a  dish  of  water 
deep  enough  to  float  them.  When  one  piece  touches  the 
other,  if  only  at  a  single  point,  regelation  instantly  sets  in. 
Thus  a  train  of  pieces  may  be  caused  to  touch  each  other, 
and,  after  they  have  once  so  touched,  you  may  take  the 
terminal  piece  of  the  train,  and,  by  means  of  it,  draw  all 
the  others  after  it.  When  we  seek  to  bend  two  pieces 
thus  united  at  their  point  of  junction,  the  frozen  points 
suddenly  separate  by  fracture,  but  at  the  same  moment 
other  points  come  into  contact,  and  regelation  sets  in  be- 
tween them.  Thus  a  wheel  of  ice  might  be  caused  to  roll 
on  an  icy  surface,  the  contacts  being  incessantly  ruptured, 
with  a  crackling  noise,  and  others  as  quickly  established  by 
regelation.  In  virtue  of  this  property  of  regelation,  ice  is 
able  to  reproduce  many  of  the  phenomena  which  are  usu- 
ally ascribed  to  viscous  bodies.* 

Here,  for  example,  is  a  straight  bar  of  ice :  I  can  by 
passing  it  successively  through  a  series  of  moulds,  each 
more  curved  than  the  last,  finally  turn  it  out  as  a  semi-ring. 
The  straight  bar  in  being  squeezed  into  the  curved  mould 
breaks,  but  by  continuing  the  pressure  new  surfaces  come 
into  contact,  and  the  continuity  of  the  mass  is  restored.  I 
take  a  handful  of  those  small  ice  fragments  and  squeeze 

*  See  note  on  the  Regelation  of  Snow  Granules  in  the  Appendix  to  this 
Lecture. 


REGELATION    OF   ICE.  205 


them  together,  they  freeze  at  their  points  of  contact  and 
now  the  mass  is  one  aggregate.  The  making  of  a  snow- 
ball, as  remarked  by  Mr.  Faraday,  illustrates  the  same  prin- 
ciple. In  order  that  this  freezing  shall  take  place,  the  snow 
ought  to  be  at  32°  and  moist.  When  below  32°  and  dry, 
on  being  squeezed  it  behaves  like  salt.  The  crossing  of 
snow-bridges  in  the  upper  regions  of  the  Swiss  glaciers  is 
often  rendered  possible  solely  by  the  regelation  of  the 
snow  granules.  The  climber  treads  the  mass  carefully, 
and  causes  its  granules  to  regelate  :  he  thus  obtains  an 
amount  of  rigidity  which,  without  the  act  of  regelation, 
would  be  quite  unattainable.  To  those  unaccustomed  to 
such  work,  the  crossing  of  snow  bridges,  spanning,  as  they 
often  do,  fissures  100  feet  and  more  in  depth,  must  appear 
quite  appalling. 

If  I  still  further  squeeze  this  mass  of  ice  fragments,  I 
bring  them  into  still  closer  proximity.  My  hand,  however, 
is  incompetent  to  squeeze  them  very  closely  together.  I 
place  them  in  this  boxwood  mould,  which  is  a  shallow  cyl- 
inder, and  placing  a  flat  piece  of  boxwood  overhead,  I  in- 
troduce both  between  the  plates  of  a  small  hydraulic  press, 
and  squeeze  the  mass  forcibly  into  the  mould.  I  now 
relieve  the  pressure  and  turn  the  substance  out  before  you  : 
it  is  converted  into  a  coherent  cake  of  ice.  I  place  it  in 
this  lenticular  cavity  and  again  squeeze  it.  It  is  crushed 
by  the  pressure,  of  course,  but  new  contacts  establish 
themselves,  and  there  you  have  the  mass  a  lens  of  ice.  I 
now  transfer  my  lens  to  this  hemispherical  cavity,  n  (fig. 
57),  and  bring  down  upon  it  a  hemispherical  protuberance, 
p,  which  is  not  quite  able  to  fill  the  cavity.  I  squeeze  the 
mass  :  the  ice,  which  a  moment  ago  was  a  lens,  is  now 
squeezed  into  the  space  between  the  two  spherical  sur- 
faces :  I  remove  the  protuberance,  and  here  I  have  the  in-  * 
terior  surface  of  a  cup  of  glassy  ice.  By  care  I  release  it 
from  the  mould,  and  there  it  is,  a  hemispherical  cup,  which 


206 


LECTUBE  VI. 


I  can  fill  with  cold  sherry,  without  the  escape  of  a  drop. 
I  scrape  with  a  chisel  a  quantity  of  ice  from  this  block,  and 


Fig.  57. 


placing  the  spongy  mass  within  this  spherical  cavity,  c  (fig. 
58),  I  squeeze  it  and  add  to  it,  till  finally  I  can  bring  down 
another  spherical  cavity,  D,  upon  it,  enclosing  it  as  a  sphere 
between  both.  As  I  work  the  press  the  mass  becomes 
more  and  more  compacted.  I  add  more  material,  and 
again  squeeze  ;  by  every  such  act  the  mass  ;.s  made  harder, 
and  there  you  have  a  snow-ball  before  you.  such  as  you 
never  saw  before.  It  is  a  sphere  of  hard  translucent  ice, 
B.  Thus,  you  see,  broken  ice  can  be  compacted  together 

Fig.  58. 


by  pressure,  and  in  virtue  of  the  property  of  regelation, 
which  cements  its  touching  surfaces,  the  substance  may  be 
made  to  take  any  shape  we  please.  Were  the  experiment 
worth  the  trouble,  I  feel  satisfied  that  I  could  form  a  rope 
of  ice  from  this  block,  and  afterwards  coil  the  rope  into  a 
knot.  Nothing  of  course  can  be  easier  than  to  produce 
statuettes  of  the  substance  from  suitable  moulds. 

It  is  easy  to  understand  how  a  substance  so  endowed 


MOULDING  OF  ICE  BY  PRESSURE.         207 

can  be  squeezed  through  the  gorges  of  the  Alps — can  bend 
so  as  to  accommodate  itself  to  the  flexures  of  the  Alpine 
valleys,  and  can  permit  of  a  differential  motion  of  its  parts, 
without  at  the  same  time  possessing  a  sensible  trace  of  vis- 
cosity. The  hypothesis  of  viscosity,  first  started  by  Ren- 
du,  and  worked  out  with  such  ability  by  Prof.  Forbes,  ac' 
counts,  certainly,  for  half  the  facts.  Where  pressure 
comes  into  play,  the  deportment  of  ice  is  apparently  that 
of  a  viscous  body  ;  where  tension  comes  into  play,  the  anal- 
ogy with  a  viscous  body  ceases. 

I  have  thus  briefly  sketched  the  phenomena  of  existing 
glaciers,  as  far  as  they  are  related  to  our  present  subject ; 
but  the  scientific  explorer  of  mountain  regions  soon  meets 
with  appearances  which  carry  his  mind  back  to  a  state  of 
things  very  different  from  that  which  now  obtains.  The 
unmistakable  traces  which  they  have  left  behind  them  show 
that  vast  glaciers  once  existed  in  places,  from  which  they 
have  for  ages  disappeared.  Go,  for  example,  to  the  gla- 
cier of  the  Aar  in  the  Bernese  Alps  and  observe  its  present 
performances ;  look  to  the  rocks  upon  its  flanks  as  they  are 
at  this  moment,  rounded,  polished,  and  scarred  by  the 
moving  ice.  And  having  by  patient  and  varied  exercise 
educated  your  eye  and  judgment  in  these  matters,  w^alk 
down  the  glacier  towards  its  'end,  keeping  always  in  view 
the  evidences  of  the  glacier's  action.  After  quitting  the 
ice,  continue  your  walk  down  the  valley  towards  the  Grim- 
sel :  you  see  everywhere  the  same  unmistakable  record. 
The  rocks  which  rise  from  the  bed  of  the  valley  are  round- 
ed like  hogs'  backs  ;  these  are  the  '  roches  moutonnes  '  of 
Charpentier  and  Agassiz ;  you  observe  upon  them  the  larger 
flutings  of  the  ice,  and  also  the  smaller  scars  scratched  by 
pebbles,  which  the  glacier  held  as  emery  on  its  under  sur- 
face. All  the  rocks  of  the  Grimsel  have  been  thus  planed 
down.  Walk  down  the  valley  of  Hasli  and  examine  the 
mountain  sides  right  and  left ;  without  the  key  which  I 


208  LECTUEE  VI. 

now  suppose  you  to  possess,  you  would  be  in  a  land  of 
enigmas ;  but  with  this  key  all  is  plain,  you  see  everywhere 
the  well-known  scars  and  flutings  and  furrowings.  In  the 
bottom  of  the  valley  you  have  the  rocks  filed  down  in  some 
places  to  dome-shaped  masses,  and,  in  others,  polished  so 
smooth  that  to  pass  over  them,  even  when  the  inclination  is 
moderate,  steps  must  be  hewn.  All  the  way  down  to 
Meyringen  and  beyond  it,  if  you  wish  to  pursue  the  en- 
quiry, these  evidences  abound.  For  a  preliminary  lesson  in 
the  recognition  of  the  traces  of  ancient  glaciers  no  better 
ground  can  be  chosen  than  this. 

Similar  evidences  are  found  in  the  valley  of  the  Rhone ; 
you  may  track  them  through  the  valley  for  eighty  miles, 
and  lose  them  at  length  in  the  lake  of  Geneva.  But  on  the 
flanks  of  the  Jura,  at  the  opposite  side  of  the  Canton  de 
Vaud,  the  evidences  reappear.  All  along  these  limestone 
slopes  you  have  strewn  the  granite  boulders  of  Mont  Blanc. 
Right  and  left  also  from  the  great  Rhone  valley  the  lateral 
valleys  show  that  they  were  once  held  by  ice.  On  the 
Italian  side  of  the  Alps  the  remains  are,  if  possible,  more 
stupendous  than- on  the  northern  side.  Grand  as  are  the 
present  glaciers  to  those  who  explore  them  in  all  their 
lengths,  they  are  mere  pigmies  in  comparison  with  their 
predecessors. 

Not  in  Switzerland  alone — not  alone  in  proximity  with 
existing  glaciers — are  these  well-known  vestiges  of  the  an- 
cient ice  discernible ;  in  the  hills  of  Cumberland  they  are 
almost  as  clear  as  in  the  Alps.  Where  the  bare  rock  has 
been  exposed  for  ages  to  the  action  of  the  weather,  the 
liner  marks  have  in  most  cases  disappeared  ;  and  the  mam- 
millated  forms  of  the  rocks  are  the  only  evidences.  But 
the  removal  of  the  soil  which  has  protected  them,  often 
discloses  rock  surfaces  which  are  scarred  as  sharply,  and 
polished  as  cleanly  as  those  which  are  now  being  scratched 
and  polished  by  the  glaciers  of  the  Alps.  Round  about 


ANCIENT  GLACIERS.  209 

Scawfell  the  traces  of  the  ancient  ice  appear,  both  in 
roches  moutonnes  and  blocs  perches  /  and  there  are  ample 
facts  to  show  that  Borrodale  was  once  occupied  by  glacier 
ice.  In  North  Wales,  also,  the  ancient  glaciers  have  placed 
their  stamp  so  firmly  upon  the  rocks,  that  the  ages  which 
have  since  elapsed  have  failed  to  obliterate  even  their 
superficial  marks.  All  round  Snowdon  these  evidences 
abound.  On  the  south-west  coast  of  Ireland  also  rise  the 
Reeks  of  Magillicuddy,  which  tilt  upwards,  and  catch  upon 
their  cold  crests  the  moist  winds  of  the  Atlantic ;  precipi- 
tation is  copious,  and  rain  at  Killarney  seems  the  rule  of 
Nature.  In  this  moist  region  every  crag  is  covered  with 
rich  vegetation ;  but  the  vapours  which  now  descend  as 
mild  and  fertilising  rain,  once  fell  as  snow,  which  formed 
the  material  for  noble  glaciers.  The  Black  Valley  was 
once  filled  by  ice,  which  planed  down  the  sides  of  the  Pur- 
ple Mountain,  as  it  moved  towards  the  Upper  Lake.  The 
ground  occupied  by  this  lake  was  entirely  held  by  the  an- 
cient ice,  and  every  island  that  now  emerges  from  its  sur- 
face is  a  glacier-dome.  The  fantastic  names  which  many 
of  the  rocks  have  received  are  suggested  by  the  shapes  into 
which  they  have  been  sculptured  by  the  mighty  moulding 
plane  which  once  passed  over  them.  North  America  is  also 
thus  glaciated.  But  the  most  notable  observation  in  con- 
nection with  this  subject  is  one  recently  made  by  Dr.  Hooker 
during  a  visit  to  Syria :  he  has  found  that  the  celebrated 
cedars  of  Lebanon  grow  upon  ancient  glacier  moraines. 

To  determine  the  condition  which  permitted  of  the  for- 
mation of  those  vast  masses  of  ice  has  long  been  a  problem 
with  philosophers,  and  a  consideration  of  the  solutions 
which  have  been  offered  from  time  to  time  will  not  be  un- 
instructive.  I  have  no  new  hypothesis,  but  it  seems  pos- 
sible to  give  a  truer  direction  and  more  definite  aim  to  our 
enquiries.  The  aim  of  all  the  writers  on  this  subject,  with 
whom  I  am  acquainted,  has  been  directed  to  the  attain- 


210  LECTURE   VI. 

ment  of  cold.  Some  eminent  men  have  thought,  and  some 
still  think,  that  the  reduction  of  temperature  during  the 
glacier  epoch  was  due  to  a  temporary  diminution  of  solar 
radiation  ;  others  have  thought  that,  in  its  motion  through 
space,  our  system  may  have  traversed  regions  of  low  tem- 
perature, and  that  during  its  passage  through  these  regions, 
the  ancient  glaciers  were  produced.  Others,  with  greater 
correctness,  have  sought  to  lower  the  temperature  by  a  re- 
distribution of  land  and  water.  If  I  understand  the  writ- 
ings of  the  eminent  men  who  have  propounded  and  advo- 
cated the  above  hypotheses,  many  of  them  seem  to  have 
overlooked  the  fact,  that  the  enormous  extension  of  gla- 
ciers in  bygone  ages,  demonstrates,  just  as  rigidly,  the 
operation  of  heat  as  the  action  of  cold. 

Cold  will  not  produce  glaciers.  You  may  have  the  bit- 
terest north-east  winds  here  in  London  throughout  the 
winter  without  a  single  flake  of  snow.  Cold  must  have  the 
fitting  object  to  operate  upon,  and  this  object — the  aqueous 
vapour  of  the  air — is  the  direct  product  of  heat.  Let  us 
put  this  glacier  question  in  another  form  :  the  latent  heat, 
of  aqueous  vapour,  at  the  temperature  of  its  production  in 
the  tropics,  is  about  1,000°  Fahr.,  for  the  latent  heat  grows 
larger  as  the  temperature  of  evaporation  descends.  A 
pound  of  water  then  vaporised  at  the  equator,  has  absorbed 
1,000  times  the  quantity  of  heat  which  would  raise  a  pound 
of  the  liquid  one  degree  in  temperature.  But  the  quantity 
of  heat  which  would  raise  a  pound  of  water  one  degree 
would  raise  a  pound  of  cast-iron  ten  degrees :  hence,  simply 
to  convert  a  pound  of  the  water  of  the  equatorial  ocean 
into  vapour,  would  require  a  quantity  of  heat  sufficient  to  im- 
part to  a  pound  of  cast-iron  10,000  degrees  of  temperature. 
But  the  fusing  point  of  cast-iron  is  2,000  Fahr. ;  therefore, 
for  every  pound  of  vapour  produced,  a  quantity  of  heat  has 
been  expended  by  the  sun  sufficient  to  raise  5  Ibs.  of  cast- 
iron  to  its  melting  point.  Imagine,  then,  every  one  of 


HYPOTHESES   TO   ACCOUNT  FOE  ANCIENT   GLACIERS.     211 

those  ancient  glaciers  with  its  mass  of  ice  quintupled ;  and 
let  the  place  of  the  mass,  so  augmented,  be  taken  by  an 
equal  mass  of  cast-iron  raised  to  the  white  heat  of  fusion, 
and  we  have  the  exact  expression  of  the  solar  action  in- 
volved in  the  production  of  the  ancient  glaciers.  Substi- 
tute the  hot  iron  for  the  cold  ice — our  speculations  would 
instantly  be  directed  to  account  for  the  high  temperature 
of  the  glacial  epoch,  and  a  complete  reversal  of  some  of 
the  hypotheses  above  quoted  would  probably  ensue. 

It  is  perfectly  manifest  that  by  weakening  the  sun's  ac- 
tion, either  through  a  defect  of  emission,  or  by  the  steep- 
ing of  the  entire  solar  system  in  space  of  a  low  tempera- 
ture, we  should  be  cutting  off  the  glaciers  at  their  source. ' 
Vast  masses  of  mountain  ice  indicate,  infallibly,  commen- 
surate masses  of  atmospheric  vapour,  and  a  proportionately 
vast  action  on  the  part  of  the  sun.  In  a  distilling  appara- 
tus, if  you  required  to  augment  the  quantity  distilled,  you 
would  not  surely  attempt  to  obtain  the  low  temperature 
necessary  to  distillation,  by  taking  the  fire  from  under  your 
boiler  ;  but  this,  if  I  understand  them  aright,  is  what  has 
been  done  by  those  philosophers  who  have  sought  to  pro- 
duce the  ancient  glaciers  by  diminishing  the  sun's  heat.  It 
is  quite  manifest  that  the  thing  most  needed  to  produce  the 
glaciers  is  an  improved  condenser  /  we  cannot  afford  to  lose 
an  iota  of  solar  action  ;  wre  need,  if  anything,  more  vapour, 
but  we  need  a  condenser  so  powerful  that  this  vapour,  in- 
stead of  falling  in  liquid  showers  to  the  earth,  shall  be  so 
far  reduced  in  temperature  as  to  descend  in  snow.  The 
problem,  I  think,  is  thus  narrowed  to  the  precise  issue  on 
which  its  solution  depends. 

NOTE. 

In  moulding  ice,  it  is  advisable  to  first  wet  the  mould  with  hot  water. 
This  facilitates  the  removal  of  the  compressed  substance.  The  ice-cup, 
referred  to  in  §  234,  may  be  from  2^  to  3  inches  in  external  diameter,  but 
the  thickness  of  the  cup  ought  not  to  exceed  a  quarter  of  an  inch.  A 
conical  plug  is  inserted  into  my  own  moulds,  the  tapping  of  which  soon 
detaches  the  ice. 


APPENDIX    TO    LECTURE    VI. 


ABSTEACT  OF  A  DISCOUKSE  ON  THE  MEK-DE-GLACE.* 

A  PORTION  of  a  series  of  observations  made  upon  the  Mer-de- 
Glace  of  Chamouni  during  the  months  of  July  and  August  last 
year,  formed  the  basis  of  this  discourse. 

The  law  first  established  by  [M.  Agassiz  and]  Prof.  J.  D. 
Forbes,  that  the  central  portions  of  a  'glacier  moved  faster  than 
the  sides,  was  amply  illustrated  by  the  deportment  of  lines  of 
stakes  placed  across  the  Mer-de-Glace  at  several  places,  and  across 
the  tributaries  of  the  glacier.  The  portions  of  the  Mer-de-Glace 
derived  from  these  tributaries  were  easily  traceable  throughout 
the  glacier  by  means  of  the  moraines.  Thus,  for  example,  that 
portion  of  the  trunk  stream  derived  from  the  Glacier  du  Geant, 
might  be  distinguished,  in  a  moment,  from  the  portion  derived 
from  the  other  tributaries,  by  the  absence  of  the  debris  of  the 
moraines  upon  the  surface  of  the  former.  The  commencement  of 
the  dirt  formed  a  distinct  junction  between  both  portions.  Atten- 
tion has  been  drawn  by  Prof.  Forbes  to  the  fact,  that  the  eastern 
side  of  the  glacier  in  particular  is  *  excessively  crevassed ; '  and  he 
accounts  for  this  crevassing  by  supposing  that  the  Glacier  du 
Geant  moves  most  swiftly,  and  in  its  efforts  to  drag  its  more  slug- 
gish companions  along  with  it,  tears  them  asunder,  and  thus  pro- 
duces the  fissures  and  dislocations  for  which  the  eastern  side  of 
the  glacier  is  remarkable.  The  speaker  said  that  too  much  weight 
must  not  be  attached  to  this  explanation.  It  was  one  of  those 
suggestions  which  are  perpetually  thrown  out  by  men  of  science 

*  Given  at  the  Royal  Institution  of  Great  Britain,  on  Friday,  June  4, 
1858.  Bj  John  Tyndall,  F.R.S. 


MOTION   OF  MEK-DE-GLACE.  213 

during  the  course  of  an  investigation,  and  the  fulfillment  or  non- 
fulfillment of  which  cannot  materially  affect  the  merits  of  the  in- 
vestigator. Indeed,  the  merits  of  Forbes  must  be  judged  on  far 
broader  grounds ;  and  the  more  his  labours  are  compared  with 
those  of  other  observers,  the  more  prominently  does  his  compara- 
tive intellectual  magnitude  come  forward.  The  speaker  would 
not  content  himself  with  saying  that  the  book  of  Prof.  Forbes  was 
the  best  book  which  had  been  written  upon  the  subject.  The 
qualities  of  mind,  and  the  physical  culture  invested  in  that  excel- 
lent work,  were  such  as  to  make  it,  in  the  estimation  of  the  phys- 
ical investigator  at  least,  outweigh  all  other  books  upon  the  sub- 
ject taken  together.*  While  thus  acknowledging  its  merits,  let  a 
free  and  frank  comparison  of  its  statements  with  facts  be  insti- 
tuted. To  test  whether  the  Glacier  du  Geant  moved  quicker  than 
its  fellows,  five  different  lines  were  set  out  across  the  Mer-de- 
Glace,  in  the  vicinity  of  the  Montenvert,  and  in  each  of  these  it 
was  found  that  the  point  of  swiftest  motion  did  not  lie  upon  the 
Glacier  du  Geant  at  all ;  but  was  displaced  so  as  to  bring  it  com- 
paratively close  to  the  eastern  side  of  the  glacier.  These  measure- 
ments prove  that  the  statement  referred  to  is  untenable  ;  but  the 
deviation  of  the  point  of  swiftest  motion  from  the  centre  of  the 
glacier  will  doubtless  be  regarded  by  Prof.  Forbes  as  of  far  great- 
er importance  to  his  theory.  At  the  place  where  these  measure- 
ments were  made,  the  glacier  turns  its  convex  curvature  to  the 
eastern  side  of  the  valley,  being  concave  towards  the  Montenvert. 
Let  us  take  a  bolder  analogy  than  even  that  suggested  in  the  ex- 
planation of  Forbes,  where  he  compares  the  Glacier  du  Geant  to  a 
strong  and  swiftly-flowing  river.  Let  us  enquire  how  a  river 
would  behave  in  sweeping  round  a  curve  similar  to  that  here 
existing.  The  point  of  swiftest  motion  would  undoubtedly  lie  on 

*  Since  the  above  was  written,  my  '  Glaciers  of  the  Alps '  has  been 
published,  and,  soon  after  its  appearance,  a  *  Reply '  to  those  portions  of  the 
book  which  referred  to  the  labours  of  M.  Rendu  was  extensively  circulated 
by  Principal  Forbes.  For  more  than  two  years  I  have  abstained  from 
answering  my  distinguished  censor ;  not  from  inability  to  do  so,  but  because 
I  thought,  and  think,  that,  within  the  limits  of  the  case,  it  is  better  to  sub- 
mit to  misconception,  than  to  make  science  the  arena  of  a  purely  personal 
controversy. 


214:  APPENDIX  TO  LECTURE  VI. 

that  side  of  the  centre  of  the  stream  towards  which  it  turns  its 
convex  curvature.  Can  this  be  the  case  with  the  ice  ?  If  so,  then 
we  ought  to  have  a  shifting  of  the  point  of  maximum  motion 
towards  the  western  side  of  the  valley,  when  the  curvature  of  the 
glacier  so  changes  as  to  turn  its  convexity  to  the  western  side. 
Such  a  change  of  flexure  occurs  opposite  the  passages  called  Les 
Ponts,  and  at  this  place  the  view  just  enunciated  was  tested.  It 
was  soon  ascertained  that  the  point  of  swiftest  motion  here  lay  at 
a  different  side  of  the  axis  from  that  observed  lower  down.  But 
to  confer  strict  numerical  accuracy  upon  the  result,  stakes  were 
fixed  at  certain  distances  from  the  western  side  of  the  glacier,  and 
others  at  equal  distances  from  the  eastern  side.  The  velocities  of 
these  stakes  were  compared  with  each  other,  two  by  two  ;  a  stake 
on  the  western  side  being  always  compared  with  a  second  one, 
which  stood  at  the  same  distance  from  the  eastern  side.  The  re- 
sults of  this  measurement  are  given  in  the  following  table,  the 
numbers  denoting  inches  : — 

1st  pair  2nd  pair  3rd  pair  4th  pair  5th  pair 

West  15       West  17J      West  22£      West  23£      West  23£ 
East  12£      East   15£      East   15£      East   18£      East  19£ 

It  is  here  seen  that  in  each  case  the  western  stake  moved  more 
rapidly  than  its  eastern  fellow  stake;  thus  proving,  beyond  a 
doubt,  that  opposite  the  Fonts  the  western  side  of  the  Mer-de- 
Glace  moves  quickest — a  result  precisely  the  reverse  of  that  ob- 
served where  the  curvature  of  the  valley  was  different. 

But  another  test  of  the  explanation  is  possible.  Between  the 
Fonts  and  the  promontory  of  Trelaporte,  the  glacier  passes  a 
point  of  contrary  flexure,  its  convex  curvature  opposite  to  Trela- 
porte being  turned  towards  the  base  of  the  Aiguille  du  Moine, 
which  stands  on  the  eastern  side  of  the  valley.  A  series  of  stakes 
was  placed  across  the  glacier  here ;  and  the  velocities  of  those 
placed  at  certain  distances  from  the  western  side  were  compared, 
as  before,  with  those  of  stakes  placed  at  the  same  distances  from 
the  eastern  side.  The  following  table  shows  the  result  of  these 
measurements  ;  the  numbers,  as  before,  denote  inches  : — 

1st  pair  2nd  pair  3rd  pair 

West    .     .  12f        West    .     .15  West  17£ 

East      .     .  14f        East      .     .  17i          East   19 


MOTION   OF   MER-DE-GLACE. 


215 


Fig.  59. 


Here  we  find  that  in  each  case  the  eastern  stake  moved  faster 
than  its  fellow.  The  point  of  maximum  motion  has  therefore 
once  more  crossed  the  axis  of  the  glacier,  being  now  upon  its  east- 
ern side. 

Determining  the  points  of  maximum  motion  for  a  great  num- 
ber of  transverse  sections  of  the  Mer-de-Glace,  and  uniting  these 
points,  we  have  the  locus  of  the  curve  described  by  the  point  re- 
ferred to.  Fig.  59  represents  a  sketch  of  the  Mer-de-Glace.  The 
dotted  line  is  drawn  along  the  centre  of  the  gla- 
cier ;  the  defined  line,  which  crosses  the  axis  of 
the  glacier  at  the  points  A  A,  is  then  the  locus  of 
the  point  of  swiftest  motion.  It  is  a  curve  more 
deeply  sinuous  than  the  valley  itself,  and  crosses 
the  central  line  of  the  valley  at  each  point  of 
contrary  flexure.  The  speaker  drew  attention  to 
the  fact  that  the  position  of  towns  upon  the 
banks  of  rivers  is  usually  on  the  convex  side  of 
the  stream,  where  the  rush  of  the  water  renders 
silting-up  impossible  :  the  Thames  was  a  case  in 
point ;  and  the  same  law  which  regulated  its 
flow  and  determined  the  position  of  the  adjacent 
towns,  is  at  this  moment  operating,  with  silent 
energy,  among  the  Alpine  glaciers. 

Another  peculiarity  of  glacier  motion  is  now 
to  be  noticed. 

Before  any  observations  had  been  made  upon 
the  subject,  it  was  surmised  by  Prof.  Forbes  that 
the  portions  of  a  glacier  near  its  bed  were  retarded  by  friction 
against  the  latter.  This  view  was  afterwards  confirmed  by  his 
own  observations,  and  by  those  of  M.  Martins.  Nevertheless  the 
state  of  our  knowledge  upon  the  subject,  rendered  further  con- 
firmation of  the  fact  highly  desirable.  A  rare  opportunity  for 
testing  the  question  was  furnished  by  an  almost  vertical  precipice 
of  ice,  constituting  the  side  of  the  Glacier  de  Geant,  which  was 
exposed  near  the  Tacul.  The  precipice  was  about  140  feet  in 
height.  At  the  top  and  near  the  bottom  stakes  were  fixed,  and 
by  hewing  steps  in  the  ice,  the  speaker  succeeded  in  fixing  a 
stake  in  the  face  of  the  precipice,  at  a  point  about  40  feet  above 
the  base.  After  the  lapse  of  a  sufficient  number  of  days,  the  prog- 


216  APPENDIX  TO   LECTUEE   VI. 

ress  of  the  three  stakes  was  measured ;  reduced  to  the  diurnal 
rate,  the  motion  was  as  follows  : — 

Top  stake         .         .  G'OO  inches 
Middle  stake     .         .4*59     „ 
Bottom  stake    .         .  2'56     ,, 

We  thus  see  that  the  top  stake  moved  with  more  than  twice 
the  velocity  of  the  bottom  one  ;  while  the  velocity  of  the  middle 
stake  lies  between  the  two.  But  it  also  appears  that  the  augmen- 
tation of  velocity  upwards  is  not  proportional  to  the  distance 
from  the  bottom,  but  increases  in  a  quicker  ratio.  At  a  height 
of  100  feet  from  the  bottom,  the  velocity  would  undoubtedly  be 
practically  the  same  as  at  the  surface.  Measurements  made  upon 
an  adjacent  ice-cliff  proved  this.  We  thus  see  the  perfect  validity 
of  the  reason  assigned  by  Forbes  for  the  continued  verticality  of 
the  walls  of  transverse  crevasses.  Indeed  a  comparison  of  the  re- 
sult with  his  anticipations  and  reasonings  will  prove  alike  their 
sagacity  and  their  truth. 

The  most  commanding  view  of  the  Mer-de-Glace  and  its  trib- 
utaries is  obtained  from  a  point  above  the  remarkable  cleft  in  the 
mountain  range  underneath  the  Aiguille  de  Charmoz,  which  is 
sure  to  attract  the  attention  of  an  observer  standing  at  the  Mont- 
envert.  This  point,  which  is  marked  G  on  the  map  of  Forbes, 
the  speaker  succeeded  in  attaining.  A  Tubingen  professor  once  vis- 
ited the  glaciers  of  Switzerland,  and  seeing  these  apparently  rigid 
masses  enclosed  in  sinuous  valleys,  went  home  and  wrote  a  book, 
flatly  denying  the  possibility  of  their  motion.  An  inspection  from 
the  point  now  referred  to  would  have  doubtless  confirmed  him  in 
his  opinion ;  and  indeed  nothing  can  be  more  calculated  to  im- 
press the  mind  with  the  magnitude  of  the  forces  brought  into 
play  than  the  squeezing  of  the  three  tributaries  of  the  Mer-de- 
Glace  through  the  neck  of  the  valley  at  Trelaporte.  But  let  us 
state  numerical  results.  Previous  to  its  junction  with  its  fellows, 
the  Glacier  du  G6ant  measures  1,134  yards  across.  Before  it  is 
influenced  by  the  thrust  of  the  Talefre,  the  Glacier  de  Lechaud 
had  a  width  of  825  yards  ;  while  the  width  of  the  Talefre  branch 
across  the  base  of  the  cascade,  before  it  joins  the  Lechaud,  is  ap- 
proximately 638  yards.  The  sum  of  these  widths  is  2,597  yards. 
At  Trelaporte  those  three  branches  are  forced  through  a  gorge 


MOTION   OF  MER-DE-GLACE.  217 

893  yards  wide,  with  a  central  velocity  of  20  inches  a  day  !  The 
result  is  still  more  astonishing,  if  we  confine  our  attention  to  one 
of  the  tributaries — that  of  the  Lechaud.  Before  its  junction  with 
the  Talefre,  the  glacier  has  a  width  of  37£  English  chains.  At 
Trelaporte  this  broad  ice  river  is  squeezed  to  a  driblet  of  less 
than  4  chains  in  width — that  is  to  say,  to  about  one-tenth  of  its 
previous  horizontal  transverse  dimension. 

Whence  is  the  force  derived  which  drives  the  glacier  through 
the  gorge  ?  The  speaker  believed  that  it  must  be  a  pressure  from 
behind.  Other  facts  -also  suggest  that  the  Glacier  du  Geant  is 
throughout  its  length  in  a  state  of  forcible  longitudinal  compres- 
sion. Taking  a  scries  of  points  along  the  axis  of  this  glacier — if 
these  points,  during  the  descent  of  the  glacier,  preserved  their 
distances  asunder  perfectly  constant — there  could  be  no  longitu- 
dinal compression.  The  mechanical  meaning  of  this  term,  as  ap- 
plied to  a  substance  capable  of  yielding  like  ice,  must  be  that  the 
hinder  points  are  incessantly  advancing  upon  the  forward  ones. 
The  speaker  was  particularly  anxious  to  test  this  view,  which  first 
occurred  to  him  from  a  priori  considerations.  Three  points,  ABC, 
were  therefore  fixed  upon  the  axis  of  the  Glacier  du  Geant,  A  be- 
ing the  highest  up  the  glacier.  The  distance  between  A  and  B 
was  545  yards,  and  that  between  B  and  c  was  487  yards.  The 
daily  velocities  of  these  three  points,  determined  by  the  theodo- 
lite, were  as  follows : — 

A  .  2O55  inches 

B  .  15-43     „ 

C        .         .  12-75     „ 

The  result  completely  corroborates  the  foregoing  anticipation. 
The  hinder  points  are  incessantly,  advancing  upon  those  in  front, 
and  that  to  an  extent  sufficient  to  shorten  a  segment  of  this  gla- 
cier, measuring  1,000  yards  in  length,  at  the  rate  of  8  inches  a 
day.  Were  this  rate  uniform  at  all  seasons,  the  shortening  would 
amount  to  240  feet  in  a  year.  When  we  consider  the  compactness 
of  this  glacier,  and  the  uniformity  in  the  width  of  the  valley 
which  it  fills,  this  result  cannot  fail  to  excite  surprise ;  and  the 
exhibition  of  force  thus  rendered  manifest  must,  in  the  speaker's 
opinion,  b.c  mainly  instrumental  in  driving  the  glacier  through  the 
jaws  of  the  granite  vice  at  Trelaporte. 
10 


218  APPENDIX   TO   LECTURE   VI. 

In  virtue  of  what  quality,  then,  can  ice  be  bent  and  squeezed, 
and  change  its  form  in  the  manner  indicated  in  the  foregoing  ob- 
servations ?  The  only  theory  worthy  of  serious  consideration  at 
the  present  day  is  that  of  Prof.  Forbes,  which  attributes  these 
effects  to  the  viscosity  of  the  ice.  The  speaker  did  not  agree  with 
this  theory ;  as  the  term  viscosity  appeared  to  him  to  be  wholly 
inapplicable  as  expressive  of  the  physical  constitution  of  the  gla- 
cier ice.  He  had  already  moulded  ice  into  cups,  bent  it  into 
rings,  changed  its  form  in  a  variety  of  ways  by  artificial  pressure, 
and  he  had  no  doubt  of  his  ability  to  mould  a  compact  mass  of 
Norway  ice  which  stood  upon  the  table  into  a  statuette;  but 
would  viscosity  be  the  proper  term  to  apply  to  the  process  of 
bruising  and  regelation  by  which  this  result  could  be  attained  ? 
He  thought  not.  A  mass  of  ice  at  32°  is  very  easily  crushed,  but 
it  has  as  sharp  and  definite  a  fracture  as  a  mass  of  glass.  There 
is  no  sensible  evidence  of  viscosity. 

The  very  essence  of  viscosity  is  the  ability  of  yielding  to  a 
force  of  tension,  the  texture  of  the  substance,  after  yielding,  being 
in  a  state  of  equilibrium,  so  that  it  has  no  strai'i  to  recover  from ; 
and  the  substances  chosen  by  Prof.  Forbes,  as  illustrative  of  the 
physical  condition  of  a  glacier,  possess  this  power  of  being  drawn 
out  in  a  very  eminent  degree.  But  it  has  been  urged,  and  justly 
urged,  that  we  ought  not  to  conclude  that  viscosity  is  absent  be- 
cause hand  specimens  do  not  show  it,  any  more  than  we  ought  to 
conclude  that  ice  is  not  blue  because  small  fragments  of  the  sub- 
stance do  not  exhibit  this  colour.  To  test  the  question  of  viscos- 
ity, then,  we  must  appeal  to  the  glacier  itself.  Let  us  do  so. 
First,  an  analogy  between  the  motion  of  a  glacier  through  a  sinu- 
ous valley,  and  of  a  river  in  a  sinuous  channel,  has  been  already 
pointed  out.  But  the  analogy  fails  in  one  important  particular : 
the  river,  and  much  more  so  a  mass  of  flowing  treacle,  honey,  tar, 
or  melted  caoutchouc,  sweeps  round  its  curves  without  rupture  of 
continuity.  The  viscous  mass  stretches,  but  the  icy  mass  ~breaks,  and 
the  *  excessive  crevassing  '  pointed  out  by  Prof.  Forbes  himself  is 
the  consequence.  Secondly,  the  inclinations  of  the  Mer-de-Glace 
and  its  three  tributaries  were  taken,  and  the  association  of  trans- 
verse crevasses  with  the  changes  of  inclination  was  accurately 
noted.  Every  Alpine  traveller  knows  the  utter  dislocation  and 
confusion  produced  by  the  descent  of  the  Mer-de-Glace  from  the 


FRAGILITY  OF  ICE.  219 

Chapeau  downwards.  A  similar  state  of  things  exists  in  the  ice- 
cascade  of  the  Talefre.  Descending  from  the  Jardin,  as  the  ice 
approaches  the  fall,  great  transverse  chasms  are  formed,  which  at 
length  follow  each  other  so  speedily  as  to  reduce  the  ice  masses 
between  them  to  mere  plates  and  wedges,  along  which  the  ex- 
plorer has  to  creep  cautiously.  These  plates  and  wedges  are  in 
some  cases  bent  and  crumpled  by  the  lateral  pressure,  and  on 
some  masses  vortical  forces  appeared  to  have  acted,  turning  large 
pyramids  90°  round,  so  as  to  set  their  structure  at  right  angles  to 
its  normal  position.  The  ice  afterwards  descends  the  fall,  the 
portions  exposed  to  view  being  a  fantastic  assemblage  of  frozen 
boulders,  pinnacles,  and  towers,  some  erect,  some  leaning,  falling 
at  intervals  with  a  sound  like  thunder,  and  crushing  the  ice  crags 
on  which  they  fall  to  powder.  The  descent  of  the  ice  through 
this  outlet  has  been  referred  to  as  a  proof  of  its  viscosity ;  but 
the  description  just  given  does  not,  it  was  believed,  harmonise 
with  our  ideas  of  a  viscous  substance. 

But  the  proof  of  the  non-viscosity  of  the  substance  must  be 
sought  at  places  where  the  change  of  inclination  is  very  small. 
Nearly  opposite  1'Angle  there  is  a  change  from  4  to  9  degrees,  and 
the  consequence  is  a  system  of  transverse  fissures,  which  renders 
the  glacier  here  perfectly  impassable.  Further  up  the  glacier, 
transverse  crevasses  are  produced  by  a  change  of  inclination  from 
3  to  5  degrees.  This  change  of  inclination  is  accurately  protracted 
in  fig.  60  ;  the  bend  occurs  at  the  point  B  ;  it  is  scarcely  percep- 


tible, and  still  the  glacier  is  unable  to  pass  over  it  without  break- 
ing across.  Thirdly,  the  crevasses  are  due  to  a  state  of  strain, 
from  which  the  ice  relieves  itself  by  breaking  :  the  rate  at  which 
they  widen  may  be  taken  as  a  measure  of  the  amount  of  relief 
demanded  by  the  ice.  Both  the  suddenness  of  their  formation, 
and  the  slowness  with  which  they  widen,  are  demonstrative  of 
the  non- viscosity  of  the  ice.  For  were  the  substance  capable  of 
stretching  even  at  the  small  rate  at  which  they  widen,  there  would 
be  no  necessity  for  their  formation. 

Further,  the  marginal  crevasses  of  a  glacier  are  known  to  be  a 


220  APPENDIX  TO   LECTURE   VI. 

consequence  of  the  swifter  flow  of  its  central  portions,  which 
throws  the  sides  into  a  state  of  strain,  from  which  they  relieve 
themselves  by  breaking.  Now  it  is  easy  to  calculate  the  amount 
of  stretching  demanded  of  the  ice  in  order  to  accommodate  itself 
to  the  speedier  central  flow.  Take  the  case  of  a  glacier,  half  a 
mile  wide.  A  straight  transverse  element,  or  slice,  of  such  a  gla- 
cier, is  bent  in  twenty-four  hours  to  a  curve.  The  ends  of  the 
slice  move  a  little,  but  the  centre  moves  more  :  let  us  suppose  the 
versed  side  of  the  curve  formed  by  the  slice  in  twenty-four  hours 
to  be  a  foot,  which  is  a  fair  average.  Having  the  chord  of  this 
arch,  and  its  versed  side,  we  can  calculate  its  length.  In  the  case 
of  the  Mer-de-Glace,  which  is  about  half-a-mile  wide,  the  amount 
of  stretching  demanded  would  be  about  the  eightieth  of  an  inch 
in  twenty-four  hours.  Surely,  if  the  glacier  possessed  a  property 
which  could  with  any  propriety  be  called  viscosity,  it  ought  to 
be  able  to  respond  to  this  moderate  demand ;  but  it  is  not  able 
to  do  so :  instead  of  stretching  as  a  viscous  body,  in  obedience 
to  this  slow  strain,  it  breaks  as  an  eminently  fragile  one,  and  mar- 
ginal crevasses  are  the  consequence.  It  may  be  urged  that  it  is 
not  fair  to  distribute  the  strain  over  the  entire  length  of  the 
curve  :  but  reduce  the  distance  as  we  may,  a  residue  must  remain 
which  is  demonstrative  of  the  non-viscosity  of  the  ice. 

To  sum  up,  then,  two  classes  of  facts  present  themselves  to  the 
glacier  investigator — one  class  in  harmony  with  the  idea  of  vis- 
cosity, and  another  as  distinctly  opposed  to  it.  Where  pressure 
comes  into  play  we  have  the  former,  where  tension  comes  into  play 
we  have  the  latter.  Both  classes  of  facts  are  reconciled  by  the 
assumption,  or  rather  the  experimental  verity,  that  the  fragility 
of  ice  and  its  power  of  regelation  render  it  possible  for  it  to 
change  its  form  without  prejudice  to  its  continuity. 


NOTE  ON  THE  EEGELATION  OF  SNOW-GRANULES.* 

I  this  morning  (March  21, 1862)  noticed  an  extremely  interest- 
ing case  of  regelation.    A  layer  of  snow,  between  one  and  two 

*  Phil.  Mag.  1862,  vol.  xxiii.  p.  312. 


REGELATTON   OF   SNOW-GKANULES.  221 

inches  thick,  had  fallen  on  the  glass  roof  of  a  small  green-house 
into  which  a  door  opened  from  the  mansion  to  which  the  green- 
house was  attached.  Air,  slightly  warmed,  acting  on  the  glass 
surface  underneath,  melted  the  snow  in  immediate  contact  with 
the  glass,  and  the  layer  in  consequence  slid  slowly  down  the  glass 
roof.  The  inclination  of  the  roof  was  very  gentle,  and  the  motion 
correspondingly  gradual.  When  the  layer  overshot  the  edge  of 
the  roof,  it  did  not  drop  off,  but  bent  like  a  flexible  body  and 
hung  down  over  the  edge  for  several  inches.  The  continuity  of 
the  layer  was  broken  into  rectangular  spaces  by  the  inclined  lon- 
gitudinal sashes  of  the  roof,  and  from  local  circumstances  one  side 
of  the  roof  was  warmed  a  little  more  than  the  other :  hence  the 
subdivisions  of  the  layer  moved  with  different  velocities,  and  over- 
hung the  edge  to  different  depths.  The  bent  and  down-hanging 
layer  of  snow  in  some  cases  actually  curved  up  inwards. 

Faraday  has  shown  that  when  small  fragments  of  ice  float  on 
water,  if  two  of  them  touch  each  other,  they  instantly  cement 
themselves  at  the  point  of  contact ;  and  on  causing  a  row  of  frag- 
ments to  •  touch,  by  laying  hold  of  the  terminal  piece  of  the  row, 
you  can  draw  all  the  others  after  it.  A  similar  cementing  must 
have  taken  place  among  the  particles  of  snow  now  in  question, 
which  were  immersed  in  the  water  of  liquefaction  near  the  surface 
of  the  glass.  But  Faraday  has  also  shown  that  when  two  fragments 
of  ice  are  thus  united,  a  hinge-like  motion  sets  in  when  you  try  to 
separate  the  one  from  the  other  by  a  lateral  push  :  one  fragment 
might,  in  fact,  be  caused  to  roll  round  another,  like  a  wheel,  by 
the  incessant  rupture,  and  re-establishment  of  regelation. 

The  power  of  motion  thus  experimentally  demonstrated,  ren- 
dered it  an  easy  possibility  for  the  snow  in  question  to  bend  it- 
self in  the  manner  observed.  The  lowermost  granules,  when  the 
support  of  the  roof  had  been  withdrawn,  rolled  over  each  other 
without  a  destructionof  continuity,  and  thus  enabled  the  snow-layer 
to  bend  as  if  it  were  viscous.  The  curling  up  was  evidently  due 
to  a  contraction  of  the  inner  surface  of  the  layer,  produced,  no 
doubt,  by  the  accommodation  of  the  granules  to  each  other,  as 
they  slowly  diminished  in  size. 

J.  T. 


LECTURE    VII. 

[March  6,  1862.] 

CONDUCTION  A  TRANSMISSION  OF  MOTION — GOOD  CONDUCTORS  AND  BAD 
CONDUCTORS — CONDUCTIVITY  OF  THE  METALS  FOR  HEAT  :  RELATION  BE- 
TWEEN THE  CONDUCTIVITY  OF  HEAT  AND  THAT  OF  ELECTRICITY INFLU- 
ENCE OF  TEMPERATURE  ON  THE  CONDUCTION  OF  ELECTRICITY INFLU- 
ENCE OF  MOLECULAR  CONSTITUTION  ON  THE  CONDUCTION  OF  HEAT — RE- 
LATION OF  SPECIFIC  HEAT  TO  CONDUCTION — PHILOSOPHY  OF  CLOTHES  : 
RUMFORD'S  EXPERIMENTS — INFLUENCE  OF  MECHANICAL  TEXTURE  ON 
CONDUCTION — INCRUSTATIONS  OF  BOILERS — THE  SAFETY  LAMP — CON- 
DUCTIVITY OF  LIQUIDS  AND  GASES  t  EXPERIMENTS  OF  RUMFORD  AND 
PESPRETZ — COOLING  EFFECT  OF  HYDROGEN  GAS — EXPERIMENTS  OF  MAG- 
NUS ON  THE  CONDUCTIVITY  OF  GASES, 

I  THINK  we  are  now  sufficiently  conversant  with  our 
subject  to  distinguish  between  the  sensible  motions  pro- 
duced by  heat,  and  heat  itself.  Heat  is  not  the  clash  of 
winds ;  it  is  not  the  quiver  of  a  flame,  nor  the  ebullition 
of  water,  nor  the  rising  of  a  thermometric  column,  nor  the 
motion  which  animates  steam  as  it  rushes  from  a  boiler  in 
which  it  has  been  compressed.  All 'these  are  mechanical 
motions  into  which  the  motion  of  heat  may  be  converted  ; 
but  heat  itself  is  molecular  motion — it  is  an  oscillation 
of  ultimate  particles.  But  such  particles,  when  closely 
grouped,  cannot  oscillate  without  communication  of  motion 
from  one  to  the  other.  To  this  propagation  of  the  motion 
of  heat,  through  ordinary  matter,  we  must  this  day  devote 
our  attention. 

Here  is  a  poker,  the  temperature  of  which  I  am  scarce- 


CONDUCTION  OF  HEAT.  223 

ly  conscious  of :  I  feel  it  as  a  hard  and  heavy  T>ody,  but  it 
neither  warms  me  nor  chills  me  ;  it  has  been  before  the 
fire,  and  the  motion  of  its  particles  at  the  present  moment 
chances  to  be  the  same  as  that  which  actuates  my  nerves  ; 
there  is  neither  communication  nor  withdrawal,  and  hence 
the  temperature  of  the  poker  on  the  one  hand,  and  my 
sensations  on  the  other,  remain  unchanged.  But  I  thrust 
the  end  of  the  poker  into  the  fire  ;  it  is  heated ;  the  parti- 
cles in  contact  with  the  fire  are  thrown  into  a  state  of  more 
intense  oscillation ;  the  swinging  atoms  strike  their  neigh- 
bours, these  again  theirs,  and  thus  the  molecular  music 
rings  along  the  bar.  The  motion,  in  this  instance,  is  com- 
municated from  particle  to  particle  of  the  poker,  and  finally 
appears  at  its  most  disant  end.  If  I  now  lay  hold  of  the 
poker,  its  motion  is  communicated  to  my  nerves,  and  pro- 
duces pain  ;  the  bar  is  what  we  call  hot,  and  my  hand,  in 
popular  language,  is  burned.  Convection  we  have  already 
defined  to  be  the  transfer  of  heat,  by  sensible  masses,  from 
place  to  place  ;  but  this  molecular  transfer,  which  consists 
in  each  atom  taking  up  the  motion  of  its  neighbours,  and 
sending  it  on  to  others,  is  called  the  conduction  of  heat. 

Let  me  exemplify  this  property  of  conduction  in  a 
homely  way.  I  have  here  a  basin  filled  with  warm  water, 
and  in  the  water  I  place  this  cylinder  of  iron,  an  inch  in 

Fig.  61. 


diameter,  and  two  inches  in  height ;  this  cylinder  is  to  be 
my  source  of  heat.  I  lay  my  thermo-electric  pile,  o  (fig. 
61),  thus  flat,  with  its  naked  face  turned  upwards  and  on 


224  LECTUKE   VII. 

that  face  I  place  a  cylinder  of  copper,  c,  which  now  pos- 
sesses the  temperature  of  this  room.  We  observe  no  deflec- 
tion of  the  galvanometer.  I  now  place  my  warm  cylinder, 
i,  having  first  dried  it,  upon  the  cool  cylinder,  which  is 
supported  by  the  pile.  The  upper  cylinder  is  not  at  more 
than  a  blood  heat ;  but  you  see  that  I  have  scarcely  time 
to  make  this  remark  before  the  needle  flies  aside,  indicating 
tnat  the  heat  has  reached  the  face  of  the  pile.  Thus  the 
molecular  motion  imparted  to  the  iron  cylinder  by  the 
warm  water  has  been  communicated  to  the  copper  one, 
through  which  it  has  been  transmitted,  in  a  few  seconds, 
to  the  face  of  the  pile. 

Different  bodies  possess  different  powers  of  transmit- 
ting molecular  motion ;  in  other  words,  of  conducting 
heat.  Copper,  which  we  have  just  used,  possesses  this 
power  in  a  very  eminent  degree.  I  will  now  remove  the 
copper,  allow  the  needle  to  return  to  0°,  and  then  lay  upon 
the  face  of  the  pilexthis  cylinder  of  glass.  On  the  cylinder 
of  glass  I  place  my  iron  cylinder,  which  has  been  re-heated 
in  the  warm  water.  There  is,  as  yet,  no  motion  of  the 
needle,  and  you  would  have  to  wait  a  long  time  to  see  it 
move.  We  have  already  waited  thrice  the  time  which  the 
copper  required  to  transmit  the  heat,  and  you  see  the  needle 
continues  motionless.  I  place  cylinders  of  wood,  chalk, 
stone,  and  fireclay,  in  succession  on  the  pile,  and  heat  their 
upper  ends  in  the  same  manner ;  but  in  the  time  which  we 
can  devote  to  an  experiment,  not  one  of  these  substances  is 
competent  to  transmit  the  heat  to  the  pile.  The  molecules 
of  these  substances  are  so  hampered  or  entangled,  that 
they  are  incompetent  to  pass  the  motion  freely  from  one  to 
another.  The  bodies  are  all  bad  conductors  of  heat.  On 
the  other  hand,  I  place  cylinders  of  zinc,  iron,  lead,  bis- 
muth, &c.,  in  succession  on  the  pile ;  each  of  them,  you 
see,  has  the  power  of  transmitting  the  motion  of  heat 
swiftly  through  its  mass.  In  comparison  with  the  wood, 


CONDUCTION   OF    HEAT.  225 

stone,  chalk,  glass,  and  clay,  they  are  all  good  conductors 
of  heat. 

As  a  general  rule,  though  it  is  not  without  its  excep- 
tions, the  metals  are  the  best  conductors  of  heat.  But  the 
metals  diifer  notably  among  themselves  as '  regards  their 
powers  of  conduction.  In  illustration  of  this  I  will  com- 
pare copper  and  iron.  Here,  behind  me,  are  two  bars,  A  B, 


A  c  (fig.  62),  placed  end  to  end,  with  balls  of  wood  at- 
tached by  wax  at  equal  distances  from  the  place  of  junction. 
Under  the  junction  I  place  a  spirit-lamp,  which  heats  the 
ends  of  the  bars ;  the  heat  will  be  propagated  right  and 
left  through  both.  This  bar  is  iron,  this  one  is  copper; 
the  heat  will  travel  to  the  greatest  distance  along  the  best 
conductor,  liberating  a  greater  number  of  its  balls. 

But  for  my  present  purpose  I  want  a  quicker  experi- 
ment. Here,  then,  are  two  plates  of  metal,  the  one  of  cop- 
per, the  other  of  iron,  which  are  united  together,  so  as  to 
form  a  long  continuous  plate  c  i  (fig.  63).  To  it  a  handle 
is  attached,  which  gives  the  whole  instrument  the  shape  of 
a.  T.  From  c  to  the  middle,  the  plate  is  copper,  from  i  to 
the  middle  it  is  iron.  At  c  I  have  soldered  a  small  bar  of 
bismuth  to  the  plate ;  at  i  a  similar  bar ;  and  from  both 
bars  wires,  g  g,  lead  to  the  galvanometer.  I  warm  the 
junction  i  by  placing  my  finger  on  it ;  an  electric  cur- 
rent is  there  generated,  and  you  observe  the  deflection. 
The  red  end  of  the  needle  moves  towards  you.  I  with- 
draw my  finger,  and  the  needle  sinks  to  0°.  I  now  warm, 
in  the  same  manner,  the  junction  c  ;  the  needle  is  deflected, 
10* 


226 


LECTURE  VH. 


but  in  the  opposite  direction.  If  I  place  a  finger  on  each 
end,  at  the  same  time,  these  currents  neutralise  each  other, 
and  we  have  no  deflection.  I  now  place  a  spirit-lamp,  with 

Fig.  C3. 


a  very  small  flame,  directly  under  the  middle  of  the  com- 
pound plate  ;  the  heat  will  propagate  itself  from  the  cen- 
tre towards  the  two  ends,  passing  on  one  side  through 
copper,  and  on  the  other  through  iron.  If  the  heat  reach 
both  ends  at  the  same  instant,  the  one  end  will  neutralize 
the  other,  and  the  needle  will  rest  quiescent.  But  if  one 
end  be  reached  sooner  than  the  other,  we  shall  obtain  a 
deflection,  and  the  direction  in  which  the  needle  moves  will 
declare  which  end  is  heated.  Now  for  the  experiment :  I 
place  the  lamp  underneath,  and  in  three  seconds  the  needle 
flies  aside.  The  red  end  moves  towards  me,  which  proves 
that  the  end  c  is  heated  ;  the  molecular  motion  has  propa- 
gated itself  most  swiftly  through  the  copper.  I  allow  the 
lamp  to  remain  until  each  metal  has  taken  up  as  much  heat 
as  it  can  appropriate,  until  the  ends  of  the  plates  become 
stationary  in  temperature ;  that  is  to  say,  until  the  quan- 
tity of  heat  which  they  receive  from  the  lamp  is  exactly 
equal  to  the  quantity  dissipated  in  the  space  around  them. 
The  copper  still  asserts  its  predominance  ;  the  needle  still 
indicates  that  the  end  c  is  most  heated :  and  thus  we  prove 
copper  to  be  a  better  conductor  of  heat  than  iron.  This 
little  experiment  illustrates  how  in  natural  philosophy 
we  turn  one  agent  to  account  in  the  investigation  of  an- 


EXPERIMENTAL   ILLUSTRATIONS   OF   CONDUCTION.     227 


other.  Every  new  discovery  is  a  new  instrument :  it  was 
once  an  end,  but  it  is  soon  a  means ;  and  thus  the  growth 
of  science  is  secured. 

One  of  the  first  attempts  to  determine  with  accuracy 
the  conductivity  of  different  bodies  for  heat,  was  that  sug- 
gested by  Franklin,  and  carried  out  by  Ingenhausz.  He 
coated  a  number  of  bars  of  various  substances  with  wax, 
and  immersing  the  ends  of  the  bars  in  hot  oil,  he  observed 
the  distance  to  which  the  wax  was  melted  on  each  of  the 
bars.  The  good  conductors  melted  the  wax  to  the  great- 
est distance ;  and  the  melting  distance  furnished  a  measure 
of  the  conductivity  of  the  bar. 

The  second  method  was  that  pointed  out  by  Fourier, 
and  followed  out  experimentally  by  M.  Despretz.  A  B  (fig. 
64)  represents  a  bar  of  metal  with  holes  drilled  in  it,  in- 
tended to  contain  small  thermometers.  At  the  end  of  the 

Fig.  64. 


mill 


bar  was  placed  a  lamp  as  a  source  of  heat ;  the  heat  propa- 
gated itself  through  the  bar,  reaching  the  thermometer  a 
first,  b  next,  c  next,  and  so  on.  For  a  certain  time  the 
thermometers  continued  to  rise,  but  afterwards  the  state 
of  the  bar  became  stationary,  each  thermometer  marking  a 
constant  temperature.  The  better  the  conduction,  the 
smaller  is  the  difference  between  any  two  successive  ther- 
mometers. The  decrement,  or  fall  of  heat,  if  I  may  use 
the  term,  from  the  hot  end  towards  the  cold,  is  greater  in 
the  bad  conductors  than  in  the  good  ones,  and  from  the 


228  LECTURE  VII.    • 

decrement  of  temperature  shown  by  the  thermometers  we 
can  deduce,  and  express  by  a  number,  the  conductivity  of 
the  bar.  This  same  method  was  followed  by  MM.  Wiede- 
mann  and  Franz,  in  a  very  important  investigation,  but  in- 
stead of  using  thermometers  they  employed  a  suitable 
modification  of  the  thermo-electric  pile.  Of  the  numerous 
and  highly  interesting  results  of  these  experiments  the  fol- 
lowing is  a  resume  : — 

Conductivity 

Name  of  Substance  For  Electricity        For  Heat 

Silver  ....  100  100 

Copper         ...  73  74 

Gold    ....  59  53 

Brass   ....  22  24 

Tin      ....  23  15 

Iron     ....  13  12 

Lead    ....  11  9 

Platinum      ...  10  8 

German  Silver  6  6 

Bismuth       ...  2  2 

This  table  shows,  that,  as  regards  their  conductive 
powers,  the  metals  differ  very  widely  from  each  other. 
Calling,  for  example,  the  conductive  power  of  silver  100, 
that  of  German  silver  is  only  6.  You  may  illustrate  this 
difference  in  a  very  simple  way  by  plunging  two  spoons, 
one  of  German  silver  and  the  other  of  pure  silver,  into  the 
same  vessel  of  hot  water.  After  a  little  time  you  find  the 
free  end  of  the  silver  spoon  much  hotter  than  that  of  its 
neighbour  ;  and  if  bits  of  phosphorus  be  placed  on  the  ends 
of  the  spoons,  that  on  the  silver  will  fuse  and  ignite  in  a 
very  short  time,  while  the  heat  transmitted  through  the 
other  spoon  will  never  reach  an  intensity  sufficient  to  ignite 
the  phosphorus. 

Nothing  is  more  interesting  to  the  natural  philosopher 
than  the  tracing  out  of  connections  and  relationships  be- 
tween the  various  agencies  of  nature.  We  know  that  they 


TABLE   OF   CONDUCTIVITIES.  229 

are  a  common  brotherhood,  we  know  that  they  are  mutual- 
ly convertible,  but  as  yet  we  know  very  little  as  to  the  pre- 
cise form  of  the  conversion.  We  have  every  reason  to 
conclude  that  heat  and  electricity  are  both  modes  of  mo- 
tion ;  we  know  experimentally  that  from  electricity  we  can 
get  heat,  and  from  heat,  as  in  the  case  of  our  thermo-elec- 
tric pile,  we  can  get  electricity.  But  although  we  have,  or 
think  we  have,  tolerably  clear  ideas  of  the  character  of  the 
motion  of  heat,  our  ideas  are  very  unclear  as  to  the  precise 
nature  of  the  change  which  this  motion  must  undergo,  in 
order  to  appear  as  electricity — in  fact,  we  know  as  yet 
nothing  about  it. 

Our  table,  however,  exhibits  one  important  connection 
between  heat  and  electricity.  Beside  the  numbers  express- 
ing conductivity  for  heat,  MM.  Wiedemann  and  Franz  have 
placed  the  numbers  expressing  the  conductivity  of  the  same 
metals  for  electricity.  They  run  side  by  side :  the  good 
conductor  of  heat  is  the  good  conductor  of  electricity,  and 
the  bad  conductor  of  heat  is  the  bad  conductor  of  electri- 
city.* Thus  we  may  infer,  that  the  physical  quality  which 
interferes  with "  the  transmission  of  heat,  interferes,  in  a 
proportionate  degree,  with  the  transmission  of  electricity. 
This  common  susceptibility  of  both  forces  indicates  a  rela- 
tionship which  future  investigations  will  no  doubt  clear  up. 

Let  me  point  out  another  evidence  of  communion  be- 
tween heat  and  electricity.  I  have  here  a  length  of  wire 
made  up  of  pieces  of  two  different  kinds  of  wire ;  there 
are  three  pieces  of  platinum,  each  four  or  five  inches  long, 
and  three  pieces  of  silver  of  the  same  length  and  thickness. 
It  is  a  proved  fact  that  the  amount  of  heat  developed  in  a 
wire  by  a  current  of  electricity  of  a  certain  strength,  is  di- 
rectly proportional  to  the  resistance  of  the  wire.f  "We 

*  Professor  Forbes  had  previously  noticed  this. 
f  Joule,  Phil.  Mag.  1841,  vol.  xix.  p.  263. 


230  LECTURE   VII. 

may  figure  the  atoms  as  throwing  themselves  as  barriers 
across  the  track  of  the  electric  current — the  current  knock- 
ing against  them,  and  imparting  its  motion  to  them,  and 
rendering  the  wire  hot.  In  the  case  of  the  good  conduct- 
or, on  the  contrary,  the  current  may  be  figured  as  gliding 
freely  round  the  atoms  without  disturbing  them  in  any 
great  degree.  I  will  now  send  the  self-same  current  from 
a  battery  of  twenty  of  Grove's  cells  through  this  com- 
pound wire.  You  see  three  spaces  white-hot,  and  three 
dark  spaces  between  them.  The  white-hot  portions  of  the 
wire  are  platinum,  and  the  dark  portions  are  silver.  The 
electric  current  breaks  impetuously  upon  the  molecules  of 
the  platinum,  while  it  glides  with  little  resistance  among 
the  atoms  of  silver  thus  producing,  in  the  metals,  different 
calorific  effects.* 

Now  I  wish  to  show  you  that  the  motion  of  heat  inter- 
feres with  the  motion  of  electricity.  You  are  acquainted 
with  the  little  platinum  lamp  wrhich  stands  in  front  of  the 
table.  It  consists  simply  of  a  little  coil  of  platinum  w^ire 
suitably  attached  to  a  brass  stand.  I  can  send  a  current 
through  that  coil  and  cause  it  to  glow.  But  you  see  I  have 
introduced  into  the  circuit  two  feet  additional  of  thin  plati- 
num wire,  and  on  establishing  the  connection,  the  same 
current  passes  through  this  wire  and  the  coil.  Both,  you 
see,  are  raised  to  redness — both  are  in  a  state  of  intense 
molecular  motion.  What  I  wish  now  to  prove  is,  that  this 
motion  of  heat,  which  the  electricity  has  generated  in  these 
two  feet  of  wire,  and  in  virtue  of  which  the  wire  glows, 
offers  a  hindrance  to  the  passage  of  the  current.  The  elec- 
tricity has  raised  up  a  foe  in  its  own  path.  I  will  cool  this 
wire,  and  thereby  cause  the  heat  to  subside.  I  shall  thus 
open  a  wider  door  for  the  passage  of  the  electricity.  But 


*  May  not  the  condensed  ether  which  surrounds  the  atoms  be  the 
vehicle  of  electric  currents  ? 


CONDUCTION   OF   HEAT   AND  ELECTRICITY.  231 

if  more  electricity  passes,  it  will  announce  itself  at  the  pla- 
tinum lamp ;  it  will  raise  that  red  heat  to  whiteness,  and 
the  change  in  the  intensity  of  the  light  will  be  visible  to 
you  all. 

Fig.  65. 


Thus,  then,  I  plunge  my  red-hot  wire  into  a  beaker  of 
water  w  (fig.  65)  :  observe  the  lamp,  it  becomes  almost  too 
bright  to  look  at.  I  raise  the  wire  out  of  the  water  and 
allow  the  motion  of  heat  once  more  to  develope  itself ;  the 
motion  of  electricity  is  instantly  impeded,  and  the  lamp 
sinks  in  brightness.  I  again  dip  the  wire  into  the  cold 
water,  deeper  and  deeper  :  observe  how  the  light  becomes 
intensified — deeper  still,  so  as  to  quench  the  entire  two  feet 
of  wire  ;  the  augmented  current  raises  the  lamp  to  its  maxi- 
mum brightness,  and  now  it  suddenly  goes  out.  The  cir- 
cuit is  broken,  for  the  coil  has  actually  been  fused  by  the 
additional  flow  of  electricity. 

Let  us  now  devote  a  moment's  time  to  the  conduction  of 
cold.  To  all  appearance  cold  may  be  conducted  like  heat. 
Here  is  a  copper  cylinder,  which  I  warm  a  little  by  holding 


232  LECTURE  VH. 

it  for  a  moment  in  my  hand.  I  place  it  on  the  pile,  and  the 
needle  goes  up  to  90°,  declaring  heat.  On  this  cylinder  I 
place  a  second  one,  which,  as  you  observe,  I  have  chilled 
by  sinking  it  for  some  time  in  this  mass  of  ice.  We  wait 
a  moment,  the  needle  moves :  it  is  now  descending  to  zero, 
passes  it,  and  goes  on  to  90°  on  the  side  of  cold.  Analogy 
might  well  lead  you  to  suppose  that  the  cold  is  conducted 
downwards  from  the  top  cylinder  to  the  bottom  one,  as 
the  heat  was  conducted  in  our  former  experiments.  I  have 
no  objection  to  the  term  '  conduction  of  cold,'  if  it  be  used 
with  a  clear  knowledge  of  the  real  physical  process  in- 
volved. The  real  process  is,  that  the  warm  intermediate 
cylinder  first  delivers  up  its  motion,  or  heat,  to  the  old  cy- 
linder overhead,  and,  having  thus  lost  its  own  possession 
of  heat,  it  draws  upon  that  of  the  pile.  In  our  former  ex- 
periments we  had  conduction  of  motion  to  the  pile  ;  in  our 
present  one  we  have  conduction  of  motion  from  the  pile. 
In  the  former  case  the  pile  is  heated,  in  the  latter  chilled  ; 
the  heating  produces  a  positive  current,  the  chilling  pro- 
duces a  negative  current ;  but  it  is  in  both  cases  the  propa- 
gation of  motion  with  which  we  have  to  do,  the  heating 
and  the  chilling  depending  solely  upon  jthe  direction  of 
propagation.  I  place  one  of  these  metal  cylinders,  which  I 
have  purposely  cooled,  on  the  face  of  our  pile  ;  a  violent 
deflection  follows,  declaring  the  chilling  of  the  instrument. 
Are  we  to  suppose  the  cold  to  be  an  entity  communicated 
to  the  pile  ?  No.  The  pile  here  is  the  warm  body ;  its 
molecular  motion  is  in  excess  of  that  possessed  by  the  cyl- 
inder ;  and  when  both  come  into  contact  the  pile  seeks  to 
make  good  the  defect.  It  imparts  a  quantity  of  its  own 
motion  to  the  cylinder,  and  by  its  bounty  becomes  impov- 
erished :  it  chills  itself,  and  generates  the  current  due  to 
cold. 

I  remove  the  cold  metal  cylinder,  and  place  upon  the 
pile  a  cylinder  of  wood,  having  the  same  temperature  as 


CONDUCTION   OF   COLD.  233 

the  metal  one.  The  chill  is  very  feeble,  and  the  consequent 
deflection  very  small.  Why  does  not  the  cold  wood  pro- 
duce an  action  equal  to  that  of  the  cold  metal  ?  Simply  be- 
cause the  heat  communicated  to  it  by  the  pile  is  accumulat- 
ed at  its  under  surface  ;  it  cannot  escape  through  the  bad 
conducting  wood  as  it  escapes  through  the  metal,  and  thus 
the  quantity  of  heat  withdrawn  from  the  pile,  by  the  wood, 
is  less  than  that  withdrawn  by  the  copper.  A  similar  effect 
is  produced  when  the  human  nerves  are  substituted  for  the 
pile.  Suppose  you  come  into  a  cold  room  and  lay  your 
hand  upon  the  fire-irons,  the  chimney-piece,  the  chairs,  the 
carpet,  in  succession ;  they  appear  to  you  of  different  tem- 
peratures :  the  iron  chills  you  more  than  the  marble,  the 
marble  more  than  the  wood,  and  so  on.  Your  hand  is  affected 
exactly  as  the  pile  was  affected  in  the  last  experiment.  It 
is  needless  to  say  that  the  reverse  takes  place  when  you 
enter  a  hot  room  ;  that  is  to  say,  a  room  hotter  than  your 
own  bodies.  I  should  certainly  suffer  if  I  were  to  lie  down 
upon  a  plate  of  metal  in  a  Turkish  bath  ;  but  I  do  not  suffer 
when  I  lie  down  on  a  bench  of  wood.  By  preserving  the 
body  from  contact  with  good  conductors,  very  high  tem^ 
peratures  may  be  endured.  Eggs  may  be  boiled  and  beef- 
steaks cooked,  by  the  heat  of  an  apartment  in  which  the 
living  bodies  of  men  sustain  no  injury. 

The  exact  philosophy  of  this  last  experiment  is  worthy 
of  a  moment's  consideration.  With  it  the  names  of  Blag- 
den  and  Chantrey  are  associated,  those  eminent  men  hav- 
ing exposed  themselves,  in  ovens,  to  temperatures  consider- 
ably higher  than  that  of  boiling  water.  Let  us  compare 
the  condition  of  the  two  living  human  beings,  with  that 
of  two  marble  statues  placed  in  the  same  oven.  The  stat- 
ues become  gradually  hotter,  until  finally  they  assume  the 
temperature  of  the  air  of  the  oven ;  the  two  sculptors, 
under  the  same  circumstances,  do  not  similarly  rise  in  tem- 
perature. If  they  did,  the  tissues  of  the  body  would  be 


23tt  LECTURE  VH. 

infallibly  destroyed,  the  temperature  which  they  endured 
being  more  than  sufficient  to  stew  the  muscles  in  their  own 
liquids.  But  the  fact  is,  that  the  heat  of  the  blood  is 
scarcely  affected  by  an  augmentation  of  the  external  heat. 
This  heat,  instead  of  being  applied  to  increase  the  tem- 
perature of  the  body,  is  applied  to  the  performance  of 
work,  in  altering  the  aggregation  of  the  body ;  it  prepares 
the  perspiration,  forces  it  through  the  pores,  and  in  part 
vaporises  it.  Heat  is  here  converted  into  potential  ener- 
gy ;  it  is  consumed  in  work.  This  is  the  waste-pipe,  if  I 
may  use  the  term,  through  which  the  excess  of  heat  over- 
flows ;  and  hence  it  is,  that  under  the  most  varying  condi- 
tions of  climate  the  temperature  of  the  human  blood  is 
practically  constant.  The  blood  of  the  Laplander  is  sensi- 
bly as  warm  as  that  of  the  Hindoo  ;  while  an  Englishman, 
in  sailing  from  the  north  pole  to  the  south,  finds  his  blood- 
temperature  hardly  heightened  by  his  approach  to  the 
equator,  and  hardly  diminished  by  his  approach  to  the  ant- 
arctic pole. 

When  the  communication  of  heat  is  gradual — as  it  al- 
ways is  when  the  body  is  surrounded  by  an  imperfect 
conductor — the  heat  is  consumed  in  the  manner  indicated 
as  fast  as  it  is  supplied ;  but  if  the  supply  of  heat  be  so 
quick  (as  it  would  be  in  the  case  of  contact  with  a  good 
conductor)  that  the  conversion  into  this  harmless  potential 
energy  cannot  be  executed  with  sufficient  rapidity,  the  in- 
jury of  the  tissues  is  the  result.  Some  people  have  pro- 
fessed to  see  in  this  power  of"  the  living  body  to  resist  a 
high  temperature,  a  conservative  action  peculiar  to  the 
vital  force.  No  doubt  all  the  actions  of  the  animal  organ- 
ism are  connected  with  what  we  call  its  vitality ;  but  the 
action  here  referred  to  is  the  same  in  kind  as  the  melting 
of  ice,  or  the  vaporisation  of  water.  It  consists  simply  in 
the  diversion  of  heat  from  the  purposes  of  temperature  to 
the  performance  of  work. 


HEAT  OF   HUMAN  BODY  CONSTANT. 


235 


Thus  far  we  have  compared  the  conducting  power  of 
different  bodies  together;  but  the  same  substance  may 
possess  different  powers  of  conduction  in  different  direc- 
tions. Many  crystals  are  so  built  that  the  motion  of  heat 
runs  with  greater  facility  along  certain  lines  of  atoms  than 
along  others.  Here,  for  instance,  is  a  large  rock-crystal — 
a  crystal  of  quartz  forming  an  hexagonal  pillar,  which,  if 
complete  would  be  terminated  by  two  six-sided  pyramids. 
Heat  travels  with  greater  facility  along  the  axis  of  this 
crystal  than  across  it.  This  has  been  proved  in  a  very 
simple  manner  by  M,  de  Senarmont.  I  have  here  two 
plates  of  quartz,  one  of  which  is  cut  parallel  to  the  axis 
of  the  crystal,  and  the  other  perpendicular  to  it.  I  coat 
the  plates  with  a  layer  of  white  wax,  laid  on  by  a  camel's 
hair  pencil.  The  plates  are  pierced  at  the  centre,  and  into 
the  hole  I  inser^  °  wire,  which  I  warm  by  an  electric  cur- 


Fig. 


Fig.  67. 


rent.  B  (fig.  66)  is  the  battery  "whence  the  current  pro- 
ceeds ;  c  is  a  capsule  of  wood,  through  the  bottom  of  which 
a  sewing-needle  passes  ;  d  is  a  second  capsule,  into  which 
dips  the  point  of  the  needle,  and  Q  is  the  perforated  plate 


236  LECTURE   VH. 

of  quartz.  Each  capsule  contains  a  drop  of  mercury. 
When  the  current  passes  from  c  to  e7,  the  needle  is  heated, 
and  the  heat  is  propagated  in  all  directions.  The  wax 
melts  around  the  place  where  the  heat  is  applied ;  and  on 
this  plate,  which  is  cut  perpendicular  to  the  axis  of  the 
quartz,  I  find  the  figure  of  the  melted  wax  to  be  a  perfect 
circle  (fig.  67).  The  heat  has  travelled  with  the  same  ra- 
pidity all  round,  and  melted  the  wax  to  the  same  distance 
in  all  directions.  I  make  a  similar  experiment  with  the 
other  plate  :  the  wax  is  now  melting  ;  but  I  notice  that  its 
figure  is  no  longer  a  circle.  The  heat  travels  more  speedily 
along  the  axis  than  across  it,  and  hence  the  wax  figure  is  an 
ellipse  instead  of  a  circle  (fig.  67«).  When  the  wax  dries, 
I  will  project  magnified  images  of  these  two  plates  upon 
the  screen,  and  you  will  then  see  the  circular  figure  of  the 
melted  wax  on  the  one,  and  the  oval  figure  of  the  wax  on 
the  other.  Iceland  spar  conducts  better  along  the  crystal- 
lographic  axis  than  at  right  angles  to  it,  while  a  crystal  of 
tourmaline  conducts  best  at  right  angles  to  its  axis.  The 
metal  bismuth,  with  which  you  are  already  acquainted, 
cleaves  with  great  facility  in  one  direction,  and,  as  has  been 
well  shown  by  MM.  Svanberg  and  Matteucci,  it  conducts 
both  heat  and  electricity  better  along  the  planes  of  cleav- 
age than  across  them. 

In  wood  we  have  an  eminent  example  of  this  difference 
of  conductivity.  Upwards  of  twenty  years  ago  MM.  De 
la  Rive  and  De  Candolle  instituted  an  inquiry  into  the  con- 
ductive power  of  wood,*  and,  in  the  case  of  five  specimens 
examined,  established  the  fact  that  the  velocity  of  transmis- 
sion was  greater  along  the  fibre  than  across  it.  The  manner 
of  experiment  was  that  usually  adopted  in  inquiries  of  this 
nature,  and  which  was  applied  to  metals  by  M.  Despretz.f 

*  Mem.  de  la  Soc.  de  Geneve,  vol.  iv.  p.  70. 

f  Anriales  de  Chim.  et  de  Phys.  December  1827. 


EFFECT   OF   MOLECULAK   STRUCTURE.  237 

A  bar  of  the  substance  was  taken,  one  end  of  which  was 
brought  into  contact  with  a  source  of  heat,  and  allowed  to 
remain  so  until  a  stationary  temperature  was  assumed. 
The  temperatures  attained  by  the  bar,  at  various  distances 
from  its  heated  end,  were  ascertained  by  means  of  ther- 
mometers fitting  into  cavities  made  to  receive  them  ;  from 
these  data,  with  the  aid  of  a  well-known  formula,  the  con- 
ductivity of  the  wood  was  determined. 

To  determine  the  velocity  of  calorific  transmission  in 
different  directions  through  wood,  the  instrument  shown  in 
fig.  68  was  devised  some  years  ago  by  myself.  Q  Q'  E  R'  is 
an  oblong  piece  of  mahogany,  A  is  a  bar  of  antimony,  B  is 
a  bar  of  bismuth.  The  united  ends  of  the  two  bars  are 
kept  in  close  contact  by  the  ivory  jaws  1 1',  and  the  other 
ends  are  let  into  a  second  piece  of  ivory,  in  which  they  are 
firmly  fixed.  Soldered  to  these  ends  are  two  pieces  of 
platinum  wire,  which  proceed  to  the  little  ivory  cups  M  M, 
enter  through  the  sides  of  the  cups,  and  communicate  with 
a-  drop  of  mercury  placed  in  the  interior.  The  mahogany 
is  cut  away,  so  that  the  bars  A  and  B  are  sunk  to  a  depth 
which  places  their  upper  surfaces  a  little  below  the  general 
level  of  the  slab  of  mahogany.  The  ivory  jaws  1 1'  are  sunk 
similarly.  Two  small  projections  are  observed  in  the  figure 
jutting  from  i  i' ;  across,  from  one  projection  to  the  other, 
a  fine  membrane  is  stretched,  thus  enclosing  a  little  cham- 
ber m,  in  front  of  the  wedge-like  end  of  the  bismuth  and 
antimony  junction  ;  the  chamber  has  an  ivory  bottom,  s  is 
a  wooden  slider,  which  can  be  moved  smoothly  back  and 
forward  along  a  bevelled  groove,  by  means  of  the  lever  L. 
This  lever  turns  on  a  pivot  near  Q,  and  fits  into  a  horizontal 
slit  in  the  slider,  to  which  it  is  attached  by  the  pin  p'  pass- 
ing through  both  ;  in  the  lever  an  oblong  aperture  is  cut, 
through  which  p'  passes,  and  in  which  it  has  a  certain 
amount  of  lateral  play,  so  as  to  enable  it  to  push  the  slider 
forward  in  a  straight  line.  Two  projections  are  seen  at 


238 


LECTURE   VH. 


CONDUCTION   OF  CEYSTALS  AND   OF   WOOD.  239 

the  end  of  the  slider,  and  across,  from  projection  to  projec- 
tion, a  thin  membrane  is  stretched ;  a  chamber  m  is  thus 
formed,  bounded  on  three  sides  and  the  bottom  by  wood, 
and  in  front  by  the  membrane.  A  thin  platinum  wire,  bent 
up  and  down  several  times,  so  as  to  form  a  kind  of  grating, 
is  laid  against  the  back  of  this  chamber,  and  imbedded  in 
the  end  of  the  slider  by  the  stroke  of  a  hammer  ;  the  end 
in  which  the  wire  is  imbedded  is  then  filed  down,  until 
about  half  the  wire  is  removed,  and  the  whole  is  reduced 
to  a  uniform  flat  surface.  Against  the  common  surface  of 
the  slider  and  wire,  an  extremely  thin  plate  of  mica  is 
glued,  surncient,  simply,  to  interrupt  ah1  contact  between 
the  bent  wire  and  a  quantity  of  mercury  which  the  cham- 
ber m'  is  destined  to  contain ;  the  ends  w  w'  of  the  bent 
wire  proceed  to  two  small  cisterns  c  c',  hollowed  out  in  a 
slab  of  ivory ;  the  wires  enter  through  the  substance  into 
the  cisterns,  and  come  thus  into  contact  with  mercury,  which 
fills  the  latter.  The  end  of  the  slider  and  its  bent  wire  are 
shown  in  fig.  Q8a.  The  rectangular  space  e  f  g  h  (fig.  68) 
is  cut  quite  through  the  slab  of  mahogany,  and  a  brass 
plate  is  screwed  to  the  latter  underneath  ;  from  this  plate 
(which,  for  reasons  to  be  explained  presently,  is  cut  away, 
as  shown  by  the  dotted  lines  in  the  figure)  four  conical 
ivory  pillars  abed  project  upwards  ;  though  appearing  to 
be  upon  the  same  plane  as  the  upper  surfaces  of  the  bis- 
muth and  antimony  bars,  the  points  are  in  reality  0'3  of  an 
inch  below  the  said  surfaces. 

The  body  to  be  examined  is  reduced  to  the  shape  of  a 
cube,  and  is  placed,  by  means  of  a  pair  of  pliers,  upon  the 
four  supports  abed',  the  slider  s  is  then  drawn  up  against 
the  cube,  and  the  latter  becomes  firmly  clasped  between 
the  projections  of  the  piece  of  ivory  i  i'  on  the  one  side, 
and  those  of  the  slider  s  on  the  other.  The  chambers  m  m' 
being  filled  with  mercury,  the  membrane  in  front  of  each 
is  pressed  gently  against  the  cube  by  the  interior  fluid 


240  LECTURE   VH.  . 

mass,  and  in  this  way  perfect  contact,  which  is  absolutely 
essential,  is  secured. 

The  problem  which  requires  solution  is  the  following : 
— It  is  required  to  apply  a  source  of  heat  of  a  strictly 
measurable  character,  and  always  readily  attainable,  to 
that  face  of  the  cube  which  is  in  contact  with  the  mem- 
brane at  the  end  of  the  slider,  and  to  determine  the  quan- 
tity of  this  heat  which  crosses  the  cube  to  the  opposite 
face,  in  a  minute  of  time.  For  the  solution  of  this  prob- 
lem, two  things  are  required — first,  the  source  of  heat  to 
be  applied  to  the  left  hand  of  the  face  of  the  cube,  and  sec- 
ondly, a  means  of  measuring  the  amount  which  has  made 
its  appearance  at  the  opposite  face  at  the  expiration  of  a 
minute. 

To  obtain  a  source  of  heat  of  the  nature  described,  the 
following  method  was  adopted  : — B  is  a  small  galvanic  bat- 
tery, from  which  a  current  proceeds  to  tho  tangent  galvan- 
ometer T  ;  passes  round  the  ring  of  the  instrument,  deflect- 
ing in  its  passage  the  magnetic  needle,  which  hangs  in  the 
centre  of  the  ring.  From  T  the  current  proceeds  to  the 
rheostat  R  ;  this  instrument  consists  of  a  cylinder  of  ser- 
pentine stone,  round  which  a  German  silver  wire  is  coiled 
spirally;  by  turning  the  handle  of  the  instrument,  any 
required  quantity  of  this  powerfully  resisting  wire  is 
thrown  into  the  circuit,  the  current  being  thus  regulated  at 
pleasure.  The  sole  use  of  these  two  last  instruments,  in  the 
present  series  of  experiments,  is  to  keep  the  current  per- 
fectly constant  from  day  to  day.  From  the  rheostat  the 
current  proceeds  to  the  cistern  c,  thence  through  the  bent 
wire,  and  back  to  the  cistern  c',  from  which  it  proceeds  to 
the  other  pole  of  the  battery. 

The  bent  wire,  during  the  passage  of  the  current,  be- 
comes gently  heated  ;  this  heat  is  transmitted  through  the 
mercury  in  the  chamber  m  to  the  membrane  in  front  of 
the  chamber ;  this  membrane  becomes  the  proximate  source 


INSTRUMENTS. 

of  heat  which  is  applied  to  the  left-hand  face  of  the  cube. 
The  quantity  of  heat  transmitted  from  this  source,  through 
the  mass  of  the  cube,  to  the  opposite  face,  in  any  given 
time,  is  estimated  from  the  deflection  which  it  is  able  to 
produce  upon  the  needle  of  a  galvanometer,  connected  with 
the  bismuth  and  antimony  pair.  G  is  a  galvanometer  used 
for  this  purpose ;  from  it  proceed  wires  to  the  mercury 
cups  M  M,  which,  as  before  remarked,  are  connected  by  pla- 
tinum wires  with  A  and  B. 

The  action  of  mercury  upon  bismuth,  as  a  solvent,  is 
well  known  ;  an  amalgam  is  speedily  formed  when  the  two 
metals  come  into  contact.  To  preserve  the  thermo-electric 
couple  from  this  action,  their  ends  are  protected  by  a 
sheathing  of  the  same  membrane  as  that  used  in  front  of 
the  chambers  m  m'. 

Previous  to  the  cube's  being  placed  between  the  two 
membranes,  the  latter,  by  virtue  of  the  fluid  masses  behind 
them,  bulge  out  a  little,  thus  forming  a  pair  of  soft  and 
slightly  convex  cushions.  When  the  cube  is  placed  on  its 
supports,  and  the  slider  is  brought  up  against  it,  both  cush- 
ions are  pressed  flat,  and  thus  make  the  contact  perfect. 
The  surface  of  the  cube  is  larger  than  the  surface  of  the 
membrane  ;  *  and  thus  the  former  is  always  firmly  caught 
between  the  opposed  rigid  projections,  the  slider  being  held 
fast  in  this  position  by  means  of  the  spring  r,  which  is  then 
attached  to  the  pin  p.  The  exact  manner  of  experiment  is 
as  follows  : — JTaving  first  seen  that  the  needle  of  the  gal- 
vanometer points  to  zero,  when  the  thermo-circuit  is  com- 
plete, the  latter  is  interrupted  by  means  of  the  break-cir- 
cuit key  Jc'.  At  a  certain  moment,  marked  by  the  second- 
hand of  a  watch,  the  voltaic  circuit  is  closed  by  the  key  &, 
and  the  current  is  permitted  to  circulate  for  sixty  seconds ; 
at  the  sixtieth  second  the  voltaic  circuit  is  broken  by  the 

*  The  edge  of  each  cube  measured  0'3  inch. 

n 


2-i2  LECTURE  vn. 

left  hand  at  &,  while,  at  the  same  instant,  the  thermo-cir- 
cuit  is  closed  by  the  right  hand  at  k'.  The  needle  of  the 
galvanometer  is  instantly  deflected,  and  the  limit  of  the 
first  impulsion  is  noted ;  the  amount  of  this  impulsion  de- 
pends, of  course,  upon  the  quantity  of  heat  which  has 
reached  the  bismuth  and  antimony  junction  through  the 
mass  of  the  cube,  during  the  time  of  action.  The  limit  of 
the  first  impulsion  being  noted,  the  cube  is  removed  and 
the  instrument  is  allowed  to  cool,  until  the  needle  of  the 
galvanometer  returns  to  zero.  Another  cube  being  intro- 
duced, the  voltaic  circuit  is  once  more  closed,  the  current 
permitted  to  circulate  sixty  seconds,  then  interrupted  by 
the  left  hand,  the  thermo-circuit  being  closed  at  the  same 
moment  with  the  right,  and  the  limit  of  the  first  swing  is 
noted  as  before. 

Judging  from  the  description,  the  mode  of  experiment 
may  appear  complicated,  but  in  reality  it  in  not  so.  A  sin- 
gle experimenter  has  the  most  complete  command  over  the 
entire  arrangement.  The  wires  from  the  small  galvanic 
battery  (a  single  cell)  remain  undisturbed  from  day  to  day ; 
all  that  is  to  be  done  is  to  connect  the  battery  with  them, 
and  everything  is  ready  for  experiment. 

There  are  in  wood  three  lines,  at  right  angles  with  each 
other,  which  the  mere  inspection  of  the  substance  enables 
us  to  fix  upon  as  the  necessary  resultants  of  molecular  ac- 
tion :  the  first  line  is  parallel  to  the  fibre ;  the  second  is 
perpendicular  to  the  fibre,  and  to  the  ligneous  layers  which 
indicate  the  annual  growth  of  the  tree ;  while  the  third  is 
perpendicular  to  the  fibre,  and  parallel,  or  rather  tangential, 
to  the  layers.  From  each  of  a  number  of  trees  a  cube  was 
cut,  two  of  whose  faces  were  parallel  to  the  ligneous  lay- 
ers, two  perpendicular  to  them,  while  the  remaining  two 
were  perpendicular  to  the  fibre.  It  was  proposed  to  exam- 
ine the  velocity  of  calorific  transmission  through  the  w^ood 
in  these  three  directions.  It  may  be  remarked  that  the 


INSTRUMENTS.  24:3 

cubes  were  fair  average  specimens  of  the  woods,  and  were 
in  all  cases  well-seasoned  and  dry. 

The  cube  was  first  placed  upon  its  four  supports  a  b  c  d, 
so  that  the  line  of  flux  from  m'  to  m  was  parallel  to  the 
fibre,  and  the  deflection  produced  by  the  heat  transmitted 
in  sixty  seconds  was  observed.  The  position  of  the  cube 
was  then  changed,  so  that  its  fibre  stood  vertical,  the  line 
of  flux  from  m'  to  m  being  perpendicular  to  the  fibre,  and 
parallel  to  the  ligneous  layers  ;  the  deflection  produced  by 
a  minute's  action  in  this  case  was  also  determined.  Final- 
ly, the  cube  was  turned  90°  round,  its  fibre  being  still  ver- 
tical, so  that  the  line  of  flux  was  perpendicular  to  both  fibre 
and  layers,  and  the  consequent  deflection  was  observed. 
In  the  comparison  of  these  two  latter  directions  the  chief 
delicacy  of  manipulation  is  necessary.  It  requires  but  a 
rough  experiment  to  demonstrate  the  superior  velocity  of 
propagation  along  the  fibre,  but  the  velocities  in  all  di- 
rections perpendicular  to  the  fibre  are  so  nearly  equal  that 
it  is  only  by  great  care,  and,  in  the  majority  of  cases,  by 
numerous  experiments,  that  a  difference  of  action  can  be 
securely  established. 

The  following  table  contains  some  of  the  results  of  the 
enquiry  ;  it  will  explain  itself : — 


244: 


LECTURE  VII. 


DEFLECTIONS. 

Description  of  "Wood. 

I. 

Parallel  to 
fibre. 

II. 

Perpendicular 
to  fibre  and 
parallel  to 

ligneous  layers. 

III. 

Perpendicular 
to  fibre  and 
to 
ligneous  layers. 

j  American  Birch      

35 

9-0 

11-0 

2  Oak  

34 

95 

11-0 

3  Beech  

33 

8-8 

10-8 

4  Corornandel-wood  .. 

33 

9-8 

12-3 

5  Bird's  eye  Maple  

31 

11-0 

12-0 

31 

10-6 

12-1 

7  Box-wood  

31 

9-9 

12-0 

31 

9-9 

12-4 

9  Rose-  wood    ....         . 

31 

10-4 

12-6 

10  Peruvian-  wood  

30 

10-7 

11-7 

29 

11-4 

12-l> 

12  Walnut                

28 

11-0 

13-0 

13  Drooping  .Ash  

28 

11-0 

12-0 

28 

11-9 

13-6 

28 

10-0 

11-7 

28 

11-0 

12-1 

28 

8-6 

10-0 

18  Olive-tree  

28 

10-5 

13-2 

19  Ash        

27 

9-5 

11-5 

20  Black  Oak  

27 

8-0 

9-4 

21  Apple-tree  

2G 

100 

12-5 

26 

10-2 

12-4 

23  Chestnut   

26 

10-1 

11-5 

26 

10-6 

12-2 

25  Honduras  Mahogany  
26  Brazil-wood  

25 
25 

9-0 
11-9 

10-0 
13-9 

27  Yew  

24 

11-0 

12-0 

28  Elm                 

24 

10-0 

11-5 

24 

10-0 

12-0 

30  Portugal  Laurel     

24 

10-0 

11-5 

31  Spanish  Mahogany  

23 

11-5 

12-5 

22 

100 

12-0 

The  above  table  furnishes  us  with  a  corroboration  of 
the  result  arrived  at  by  De  la  Rive  and  De  Candolle,  re- 
garding the  superior  conductivity  of  the  wood  in  the  direc- 
tion of  the  fibre.  Evidence  is  also  afbrded  as  to  how  little 


AXES   OF   CONDUCTION   IN   WOOD.  245 

mere  density  affects  the  velocity  of  transmission.  There 
appears  to  be  neither  law  nor  general  rule  here.  American 
Birch,  a  comparatively  light  wood,  possesses  undoubtedly 
a  higher  transmissive  power  than  any  other  in  the  list. 
Iron-wood,  on  the  contrary,  with  a  specific  gravity  of  1*426, 
stands  low.  Again,  Oak  and  Coromandel-wood — the  latter 
so  hard  and  dense  that  it  is  used  for  sharp  war-instruments 
by  savage  tribes — stand  near  the  head  of  the  list,  while 
Scotch  Fir  and  other  light  woods  stand  low. 

If  we  cast  our  eyes  along  the  second  and  third  columns 
of  the  table,  we  shall  find  that  in  every  instance  the  velocity 
of  propagation  is  greatest  in  a  direction  perpendicular  to 
the  ligneous  layers.  The  law  of  molecular  action,  as  re- 
gards the  transmission  of  heat  through  wood,  may  there- 
fore be  expressed  as  follows : — 

At  all  the  points  not  situate  in  the  centre  of  the  tree, 
wood  possesses  three  unequal  axes  of  calorific  conduction, 
which  are  at  right  angles  to  each  other.  The  first,  and 
principal  axis,  is  parallel  to  the  fibre  of  the  wood ;  the 
second,  and  intermediate  axis,  is  perpendicular  to  the  fibre 
and  to  the  ligneous  layers  ;  while  the  third  and  least  axis 
is  perpendicular  to  the  fibre  and  parallel  to  the  layers. 

MM.  De  la  Rive  and  De  Candolle  have  remarked  upon 
the  influence  which  its  feeble  conducting  power  in  a  lateral 
direction  must  exert  in  preserving  within  a  tree  the  warmth 
which  it  acquires  from  the  soil.  In  virtue  of  this  property 
a  tree  is  able  to  resist  sudden  changes  of  temperature  which 
would  probably  be  prejudicial  to  it :  it  resists  alike  the  sud- 
den abstraction  of  heat  from  within  and  the  sudden  acces- 
sion of  it  from  without.  But  Nature  has  gone  further,  and 
clothes  the  tree  with  a  sheathing  of  worse-conducting  ma- 
terial than  the  wood  itself,  even  in  its  worst  direction. 
The  following  are  the  deflections  obtained  by  submitting 


246  LECTURE  vn. 

a  number  of  cubes  of  bark,  of  the  same  size  as  the  cubes 
of  wood,  to  the  same  conditions  of  experiment : — 

Deflection        Corresponding  deflection 
produced  by  the  wood 

Beech-tree  Bark  .         .         .         .7°  10'8° 

Oak-tree  Bark  ....     7  11-0 

Elm-tree  Bark  ....     7  11-5 

Pine-tree  Bark  ....     7  12'0 

The  direction  of  transmission,  in  these  cases,  was  from 
the  interior  surface  of  the  bark  outwards. 

The  average  deflection  produced  by  a  cube  of  wood, 
when  the  flux  is  lateral,  may  be  taken  at 

12°; 

a  cube  of  rock  crystal  (pure  silica),  of  the  same  size,  pro- 
duces the  deflection  of 

90°. 

Two  bodies  so  diverse,  where  they  cover  any  consider- 
able portion  of  the  earth's  surface,  must  affect  the  climate 
very  differently.  There  are  the  strongest  experimental 
grounds  for  believing  that  rock-crystal  possesses  a  higher 
conductive  power  than  some  of  the  metals. 

The  following  numbers  express  the  transmissive  power 
of  a  few  other  organic  structures  :  cubes  of  the  substances 
were  examined  in  the  usual  manner : — 

Tooth  of  Walrus        .         .         .         .16 
Tusk  of  East-Indian  Elephant     .         .17 

Whalebone 9 

Rhinoceros'-horn        ....       9 
Cow's-horn 9 

Sudden  changes  of  temperature  are  prejudicial  to  ani- 
mal and  vegetable  health ;  the  substances  used  in  the  con- 
struction of  organic  tissues  are  exactly  such  as  are  best  cal- 
culated to  resist  those  changes. 

The  following  results  further  illustrate  this  point.   Each 


LOW   CONDUCTIVITY   OF   ORGANIC   TISSUES.  247 

of  the  substances  mentioned  was  reduced  to  the  cubical 
form,  and  submitted  to  an  examination  similar  in  every 
respect  to  that  of  wood  and  quartz.  While,  however,  a 
cube  of  the  latter  substance  produces  a  deflection  of  90°,  a 
cube  of 

Sealing-wax  produces  a  deflection  of        .        .0° 

Sole  leather 0 

Becs'-wax 0 

Glue 0 

Gutta-percha 0 

India-rubber .0 

Filbert-kernel 0 

Almond-kernel 0 

Boiled  ham-muscle  .         .         .         .         .         .0 

•Raw  veal-muscle 0 

The  substances  here  named  arc  animal  and  vegetable 
productions ;  and  the  experiments  demonstrate  the  extreme 
imperviousness  of  every  one  of  them.  Starting  from  the 
principle  that  sudden  accessions  or  deprivations  of  heat  are 
prejudicial  to  animal  and  vegetable  health,  we  see  that  the 
materials  chosen  are  precisely  those  which  are  best  calcu- 
lated to  avert  such  changes. 

I  wish  now  to  direct  your  attention  to  what  may,  at 
first  sight,  appear  to  you  a  paradoxical  experiment.  Here 
is  a  short  prism  of  bismuth,  and  here  another  of  iron,  of 
the  same  size.  I  coat  the  ends  of  both  prisms  with  white 
wax,  and  then  place  them,  with  their  coated  surfaces  up- 
wards, on  the  lid  of  this  vessel,  which  contains  hot  water. 
The  motion  of  heat  will  propagate  itself  through  the 
prisms,  and  you  are  to  observe  the  melting  of  the  wax.  It 
is  already  beginning  to  yield,  but  on  which  ?  On  the  bis- 
muth. And  now  the  white  has  entirely  disappeared  from 
the  bismuth,  the  wax  overspreads  it  in  a  transparent  liquid 
layer,  while  the  wax  on  the  iron  is  not  yet  melted.  How 
is  this  result  to  be  reconciled  with  the  fact  stated  in  our 


248  LECTURE   VII. 

table  (page  224),  that,  the  conduction  of  iron  being  12, 
conduction  of  bismuth  is  only  2  ?  In  this  experiment  the 
bismuth  seems  to  be  the  best  conductor.  We  solve  this 
enigma  by  turning  to  our  table  of  specific  heat  (Lecture 
V.)  ;  we  there  find  that,  the  specific  heat  of  iron  being 
1138,  that  of  bismuth  is  only  308  ;  to  raise  it,  therefore,  a 
certain  number  of  degrees  in  temperature,  iron  requires 
more  than  three  times  the  absolute  quantity  of  heat  re- 
quired by  bismuth.  Thus,  though  the  iron  is  really  a  much 
better  conductor  than  the  bismuth,  and  is  at  this  moment 
accepting,  in  every  unit  of  time,  a  much  greater  amount 
of  heat  than  the  bismuth,  still,  in  consequence  of  the  num- 
ber of  its  atoms,  or  the  magnitude  of  its  interior  work,  the 
augmentation  of  temperature,  in  the  case  of  iron,  is  slow. 
Bismuth,  on  the  contrary,  can  immediately  devote  a  largu 
proportion  of  the  heat  imparted  to  it  to  the  augmentation 
of  temperature  ;  and  thus  it  apparently  outs  crips  the  iron  in 
the  transmission  of  that  motion  to  which  temperature  is 
due. 

You  see  here  very  plainly  the  incorrectness  of  the 
statements  sometimes  made  in  books,  and  certainly  made 
very  frequently  by  candidates  in  our  science  examinations, 
regarding  the  experiment  of  Ingenhausz,  to  which  I  have 
already  referred.  It  is  usually  stated,  that  the  greater  the 
quickness  with  which  the  wax  melts,  the  better  is  the  con- 
ductor. If  the  bad  conductor  and  the  good  conductor  have 
the  same  specific  heat,  this  is  true,  but  in  other  cases,  as 
proved  by  our  last  experiment,  it  may  be  entirely  incorrect. 
The  proper  way  of  proceeding,  as  already  indicated,  is  to 
wait  until  both  the  iron  and  the  bismuth  have  attained  a 
constant  temperature — till  each  of  them,  in  fact,  has  ac- 
cepted, and  is  transmitting,  all  the  motion  which  it  can  ac- 
cept, or  transmit,  from  the  source  of  heat ;  when  this  is 
done,  it  is  found  that  the  quantity  transmitted  by  the  iron 
is  six  times  greater  than  that  transmitted  by  the  bismuth. 


INFLUENCE   OF    SPECIFIC    HEAT.  24:9 

You  remember  our  experiments  with  the  Trevelyan  instru- 
ment, and  know  the  utility  of  having  a  highly  expansible 
body  as  the  bearer  of  the  rocker.  Lead  is  good,  because 
it  is  thus  expansible.  But  the  coefficient  of  expansion  of 
zinc  is  slightly  higher  than  that  of  lead  ;  still  zinc  does  not 
answer  well  as  a  block.  The  reason  is,  the  specific  heat  of 
zinc  is  more  than  three  times  that  of  lead,  so  that  the  heat 
communicated  to  the  zinc  by  the  contact  of  the  rocker, 
produces  only  about  one-third  the  augmentation  of  tem- 
perature, and  a  correspondingly  small  amount  of  local  ex- 
pansion. 

These  considerations  also  show  that  in  our  experiments 
on  wood  the  quantity  of  heat  transmitted  by  our  cube  in 
one  minute's  time,  cannot,  in  strictner.,  be  regarded  as  the 
expression  of  the  conductivity  of  the  wood,  unless  the 
specific  heat  of  the  various  woods  be  the  same.  On  this 
point  no  experiments  have  been  made.  But  as  regards  the 
influence  of  molecular  structure,  the  experiments  hold 
good,  for  here  we  compare  one  direction  with  another,  in 
the  same  cube.  With  respect  to  organic  structures,  I  may 
add  that,  even  allowing  them  time  to  accept  all  the  motion 
which  they  are  capable  of  accepting,  from  a  source  of  heat, 
their  power  of  transmitting  that  motion  is  exceedingly 
low.  They  are  really  bad  conductors. 

It  is  the  imperfect  conductibility  of  woollen  textures 
which  renders  them  so  eminently  fit  for  clothing.  They 
preserve  the  body  from  sudden  accessions  or  losses  of  heat. 
The  same  quality  of  non-conductibility  manifests  itself  when 
we  wrap  flannel  round  a  block  of  ice.  The  ice  thus  pre- 
served is  not  easily  melted.  In  the  case  of  a  human  body 
on  a  cold  day,  the  woollen  clothing  prevents  the  transmis- 
sion of  motion  from  within  outwards ;  in  the  case  of  the 
ice  on  a  warm  day,  the  self-same  fabric  prevents  the  trans- 
mission of  motion  from  without  inwards.  Animals  which 
inhabit  cold  climates  are  furnished  by  Nature  with  their 
11* 


250  LECTTJKE   VII. 

necessary  clothing.  Birds  especially  need  this  protection, 
for  they  are  still  more  warm-blooded  than  the  mammalia. 
They  are  furnished  with  feathers,  and  between  the  feathers 
the  interstices  are  filled  writh  -down,  the  molecular  consti- 
tution and  mechanical  texture  of  which  render  it,  perhaps, 
the  worst  of  all  conductors.  Here  we  have  another  exam- 
ple of  that  harmonious  relation  of  life  to  the  conditions  of 
life,  which  is  incessantly  presented  to  the  student  of  nat- 
ural science. 

The  indefatigable  Rumford  made  an  elaborate  series 
of  experiments  on  the  conductivity  of  the  substances  used  in 
clothing.*  His  method  was  this  : — A  mercurial  thermom- 
eter was  suspended  in  the  axis  of  a  cylindrical  glass  tube 
ending  with  a  globe,  in  such  a  manner  that  the  centre  of 
the  bulb  of  the  thermometer  occupied  the  centre  of  the 
globe  ;  the  space  between  the  internal  surface  of  the  globe 
and  the  bulb  was  filled  with  the  substance  whose  conduct- 
ive power  was  to  be  determined ;  the  instrument  was  then 
heated  in  boiling  water,  and  afterwards,  being  plunged  into 
a  freezing  mixture  of  pounded  ice  and  salt,  the  times  of 
cooling  down  135°  Fahr.  were  noted.  They  are  recorded 
in  the  following  table  : — 

Surrounded  with  Seconds 

Twisted  silk        .         .         .         .917 

Fine  lint 1032 

Cotton  wool        .         .         .         .1046 
Sheep's  wool       .  .        .  1118 

Taffety 1169 

Raw  silk 1264 

Beavers'  fur        .        .         .         .1296 
Eider  down         ....  1305 

Hares'  fur 1312 

Wood  ashes        .         .         .         .927 

Charcoal 937 

Lamp-black         .         .         .         .1117 

*  Phil.  Trans.  1792,  p.  48. 


ACTION   OF   CLOTHING.  251 

Among  the  substances  here  examined,  hares'  fur  offered 
the  greatest  impediment  to  the  transmission  of  the  heat. 

The  transmission  of  heat  is  powerfully  influenced  by 
the  mechanical  state  of  the  body  through  which  it  passes. 
The  raw  and  twisted  silk  of  Rumford's  table  illustrate 
this.  Pure  silica,  in  the  state  of  hard  rock-crystal,  is  a 
better  conductor  than  bismuth  or  lead ;  but  if  the  crystal 
be  reduced  to  powder,  the  propagation  of  heat  through 
that  powder  is  exceedingly  slow.  Through  transparent 
rock-salt  heat  is  copiously  conducted,  through  common 
table-salt  very  feebly.  I  have  here  some  asbestos,  which 
is  composed  of  certain  silicates  in  a  fibrous  condition ;  I 
place  it  on  my  hand,  and  on  it  I  place  a  red-hot  iron  ball : 
you  see  I  can  support  the  ball  without  inconvenience.  The 
asbestos  intercepts  the  ,heat.  That  this  division  of  the  sub- 
stance should  interfere  with  the  transmission  might  reason- 
ably be  inferred ;  for,  heat  being  motion,  anything  which 
disturbs  the  continuity  of  the  molecular  chain,  along  which 
the  motion  is  conveyed,  must  affect  the  transmission.  In 
the  case  of  the  asbestos  the  fibres  of  the  silicates  are  sepa- 
rated from  each  other  by  spaces  of  air ;  to  propagate  itself, 
therefore,  the  motion  has  to  pass  from  the  silicate  to  the 
air,  a  very  light  body,  and  again  from  the  air  to  the  sili- 
cate, a  comparatively  heavy  body ;  and  it  is  easy  to  see 
that  the  transmission  of  motion  through  this  composite  tex- 
ture must  be  very  imperfect.  In  the  case  of  an  animal's 
fur,  this  is  more  especially  the  case  ;  for  here  not  only  do 
spaces  of  air  intervene  between  the  hairs,  but  the  hairs 
themselves,  unlike  the  fibres  of  the  asbestos,  are  very  bad 
conductors.  Lava  has  been  known  to  flow  over  a  layer  of 
ashes  underneath  which  was  a  bed  of  ice,  and  the  non-con- 
ductivity of  the  ashes  has  saved  the  ice  from  fusion.  Red- 
hot  cannon-balls  may  be  wheeled  to  the  gun's  mouth  in 
wooden  barrows  partially  filled  with  sand.  Ice  is  packed 
in  sawdust  to  prevent  it  from  melting ;  powdered  charcoal 


252  LECTURE  VII. 

is  also  an  eminently  bad  conductor.  But  there  are  cases 
where  sawdust,  chaff,  or  charcoal  could  not  be  used  with 
safety,  on  account  of  their  combustible  nature.  In  such 
cases,  powdered  gypsum  may  be  used  with  advantage  ;  in 
the  solid  crystalline  state  it  is  incomparably  a  worse  con- 
ductor than  silica,  and  it  may  be  safely  inferred,  that  in 
the  powdered  state  its  imperviousness  far  transcends  that 
of  sand,  each  grain  of  which  is  a  good  conductor.  A 
jacket  of  gypsum  powder  round  a  steam  boiler  would  ma- 
terially lessen  its  loss  of  heat. 

Water  usually  holds  certain  minerals  in  solution.  In 
percolating  through  the  earth,  it  dissolves  more  or  less  of 
the  substances  with  which  it  comes  into  contact.  For 
example,  in  chalk  districts  the  water  always  contains  a 
quantity  of  carbonate  of  lime ;  such  water  is  called  hard 
icater.  Sulphate  of  lime  is  also  a  common  ingredient  of 
water.  In  evaporating,  the  water  is  only  driven  off,  the  min- 
eral is  left  behind,  and  often  in  quantities  too  great  to  be 
held  in  solution  by  the  water.  Many  springs  are  strongly 
impregnated  by  carbonate  of  lime,  and  the  consequence  is, 
that  when  the  waters  of  such  springs  reach  the  surface  and 
are  exposed  to  the  air,  where  they  can  partially  evaporate, 
the  mineral  is  precipitated,  and  forms  incrustations  on  the 
surfaces'  of  plants  and  stones  over  which  the  water  trickles. 
In  the  boiling  of  water  the  same  occurs ;  the  minerals  are 
precipitated,  and  there  is  scarcely  a  kettle  in  London  which 
is  not  internally  coated  with  a  mineral  incrustation.  This 
is  an  extremely  serious  difficulty  as  regards  steam  boilers  ; 
the  crust  is  a  bad  conductor,  and  it  may  become  so  thick  as 
materially  to  intercept  the  passage  of  heat  to  the  water.  I 
have  here  an  example  of  this  mischief.  This  is  a  portion 
of  a  boiler  belonging  to  a  steamer,  which  was  all  but  lost 
through  the  exhaustion  of  her  coals :  to  bring  this  vessel 
into  port  her  spars  and  every  piece  of  available  wood  were 
burnt.  On  examination  this  formidable  incrustation  was 


WITHDRAWAL   OF   HEAT   BY   GOOD   CONDUCTORS.       253 

found  within  the  boiler :  it  is  mainly  carbonate  of  lime, 
which  by  its  non-conducting  power  rendered  a  prodigal  ex- 
penditure of  fuel  necessary  to  generate  the  required  quan- 
tity of  steam.  Doubtless  the  slowness  of  many  kettles  in 
boiling  would  be  found  due  to  a  similar  cause. 

I  wish  now  to  bring  before  you  one  or  two  instances  of 
the  action  of  good  conductors  in  preventing  the  local  ac- 
cumulation of  heat.  I  have  here  two  spheres  of  the  same 
size,  both  covered  closely  with  white  paper.  One  of  them 
is  copper,  the  other  is  wood.  I  place  a  spirit  lamp  under- 
neath each  of  them,  and  after  a  time  we  will  observe  the 
effect.  The  motion  of  heat  is,  of  course,  communicating  it- 
self to  each  ball,  but  in  one  it  is  quickly  conducted  away 
from  the  place  of  contact  with  the  flame,  through  the  entire 
mass  of  the  ball ;  in  the  other  this  quick  conduction  docs 
not  take  place,  the  motion  therefore  accumulates  at  the 
point  where  the  flame  plays  upon  the  ball ;  and  here  you 
have  the  result.  I  turn  up  the  wooden  ball,  the  white  pa- 
per is  quite  charred ;  I  turn  up  the  other  ball, — so  far  from 
being  charred,  it  is  iDet  at  its  under  surface  by  the  condensa- 
tion of  the  aqueous  vapour  generated  by  the  lamp.  Here 
is  a  cylinder  covered  closely  with  paper ;  I  hold  its  centre 
thus  over  the  lamp,  turning  it  so  that  the  flame  shall  play 
all  round  the  cylinder  :  you  see  a  well-defined  black  mark, 
on  one  side  of  which  the  paper  is  charred,  on  the  other 
side  not.  The  cylinder  is  half  brass  and  half  wood,  and 
this  black  mark  shows  their  line  of  junction :  where  the 
paper  covers  the  wood,  it  is  charred ;  where  it  covers  the 
brass,  it  is  not  sensibly  affected. 

If  the  entire  moving  force  of  a  common  rifle  bullet 
were  communicated  to  a  heavy  cannon-ball,  it  would  pro- 
duce in  the  latter  a  very  small  amount  of  motion.  Sup- 
posing the  rifle  bullet  to  weigh  two  ounces,  and  to  have  a 
velocity  of  1,600  feet  a  second,  the  moving  force  of  this 
bullet  communicated  to  a  100  Ib.  cannon-ball  would  impart 


254:  LECTURE   VII. 

to  the  latter  a  velocity  of  only  32  feet  a  second.  Thus 
with  regard  to  a  flame ;  its  molecular  motion  is  very  in- 
tense, but  its  weight  is  extremely  small,  and  if  communi- 
cated to  a  heavy  body,  the  intensity  of  the  motion  must 
fall.  For  example,  I  have  here  a  sheet  of  wire  gauze,  with 
meshes  wide  enough  to  allow  air  to  pass  through  them 
with  the  utmost  freedom  ;  and  here  is  a  jet  of  gas  burning 
brilliantly.  I  bring  down  the  wire  gauze  upon  the  flame ; 
you  would  imagine  that  the  flame  could  readily  pass 
through  the  meshes  of  the  gauze ;  but  no,  not  a  flicker  gets 
through  (fig.  69).  The  combustion  is  entirely  confined  to 


Fig.  69.  Fig.  70. 


the  space  under  the  gauze.  I  extinguish  the  flame,  and  al- 
low the  unignited  gas  to  stream  from  the  burner.  I  place 
the  wire  gauze  thus  above  the  burner :  the  gas,  I  know,  is 
now  freely  passing  through  the  meshes.  I  ignite  the  gas 
above  ;  there  you  have  the  flame,  but  it  does  not  propagate 
itself  downwards  to  the  burner  (fig.  70).  You  see  a  dark 
space  of  four  inches  between  the  burner  and  the  gauze,  a 
space  filled  with  gas  in  a  condition  eminently  favourable  to 
ignition,  but  still  it  does  not  ignite.  Thus,  you  see,  this 
metallic  gauze,  which  allows  the  gas  to  pass  freely  through, 
intercepts  the  flame.  And  why  ?  A  certain  heat  is  neces- 
sary to  cause  the  gas  to  ignite ;  but  by  placing  the  wire 
gauze  over  the  flame,  or  the  flame  over  the  wire  gauze,  you 
transfer  the  motion  of  that  light  and  quivering  thing  to 


THE   SAFETY-LAMP.  255 

the  comparatively  heavy  gauze.  The  intensity  of  the  mole- 
cular motion  is  greatly  lowered  by  being  communicated 
to  so  great  a  mass  of  matter — so  much  lowered,  indeed, 
that  it  is  incompetent  to  propagate  the  combustion  to  the 
opposite  side  of  the  gauze. 

We  are  all,  unhappily,  too  well  acquainted  with  the  ter- 
rible accidents  that  occur  through  explosions  in  coal  mines. 
You  know  that  the  cause  of  these  explosions  is  the  presence 
of  a  certain  gas — a  compound  of  carbon  and  hydrogen — 
generated  in  the  coal  strata.  When  this  gas  is  mixed  with 
a  sufficient  quantity  of  air,  it  explodes  on  ignition,  the  car- 
bon of  the  gas  uniting  with  the  oxygen  of  the  air,  to  pro- 
duce carbonic  acid ;  the  hydrogen  of  the  gas  uniting  with 
the  oxygen  of  the  air  to  produce  water.  By  the  flame  of 
the  explosion  the  miners  are  burnt ;  but  even  should  this 
not  destroy  life,  they  are  often  suffocated  afterwards  by 
the  carbonic  acid  produced.  The  original  gas  is  the  miner's 
*  fire-damp,'  the  carbonic  acid  is  his  '  choke-damp.'  Sir 
Humphry  Davy,  after  having  assured  himself  of  the  action 
of  wire  gauze,  which  I  have  just  exhibited  before  you,  ap- 
plied it  to  the  construction  of  a  lamp  which  should  enable 
the  miner  to  carry  his  light  into  an  explosive  atmosphere. 
Previous  to  the  introduction  of  the  safety-lamp,  the  miner 
had  to  content  himself  with  the  light  from  sparks  pro* 
duced  by  the  collision  of  flint  and  steel,  for  it  was  found 
that  these  sparks  were  incompetent  to  ignite  the  fire- 
damp. 

Davy  surrounded  a  common  oil  lamp  by  a  cylinder  of 
wire  gauze  (fig.  71).  As  long  as  this  lamp  is  fed  by  pure 
air,  the  flame  burns  with  the  ordinary  brightness  of  an  oil- 
flame  ;  but  when  the  miner  conies  into  an  atmosphere  which 
contains  '  fire-damp,'  his  flame  enlarges,  and  becomes  less 
luminous  ;  instead  of  being  fed  by  the  pure  oxygen  of  the 
air,  it  is  now  in  part  surrounded  by  inflammable  gas.  This 
he  ought  to  take  as  a  warning  to  retire.  Still,  though  a 


256 


LECTUKE   VII. 


continuous  explosive  atmosphere  may  extend  from  the  air 
outside,  through  the  meshes  of  the  gauze,  to  the  flame 
within,  the  ignition  is  not  propagated 
across  the  gauze.  The  lamp  may  be  filled 
with  an  almost  lightless  flame,  and  still 
explosion  does  not  occur.  A  defect  in 
the  gauze,  the  destruction  of  the  wire  at 
any  point  by  oxidation,  hastened  by  the 
flame  playing  against  it,  would  cause  an 
explosion.  The  motion  of  the  lamp 
through  the  air  might  also  force,  mechan- 
ically, the  flame  through  the  meshes.  In 
short,  a  certain  amount  of  intelligence 
and  caution  is  necessary  in  using  the 
lamp.  The  intelligence,  unhappily,  is  not 
always  possessed,  nor  the  caution  always 
exercised,  by  the  miner ;  and  the  conse- 
quence is,  that  even  with  the  safety-lamp, 
explosions  still  occur.  Before  permitting 
a  man  or  a  boy  to  enter  a  mine,  would  it 
not  be  well  to  place  these  results,  by  ex- 
periment, visibly  before  him  ?  Mere  ad- 
vice will  not  enforce  caution ;  but  let  the  miner  have  the 
physical  image  of  what  he  is  to  expect,  clearly  and  vividly 
before  his  mind,  and  he  will  find  it  a  restraining  and  a 
monitory  influence,  long  after  the  effect  of  cautioning 
words  has  passed  away. 

A  word  or  two  now  on  the  conductivity  of  liquids  and 
gases.  Rumford  made  numerous  experiments  on  this  sub- 
ject, showing  at  once  clearness  of  conception  and  skill  of 
execution.  He  supposed  liquids  to  be  non-conductors, 
clearly  distinguishing  the  c  transport '  of  heat  by  convec- 
tion from  true  conduction  ;  and  in  order  to  prevent  convec- 
tion in  his  liquids,  he  heated  them  at  the  top.  In  this  way 
he  found  the  heat  of  a  warm  iron  cylinder  incompetent  to 


us/? 


CHILLING   BY   HYDEOGEN  AND   AIE.  257 

pass  downwards  through  0-2  of  an  inch  of  olive  oil ;  he  also 
boiled  water  in  a  glass  tube,  over  ice,  without  melting  the 
substance.  The  later  experiments  of  M.  Despretz  show, 
however,  that  liquids  possess  true,  though  extremely  feeble, 
powers  of  conduction.  Rumford  also  denied  the  conductiv- 
ity of  gases,  though  he  was  well  acquainted  with  their  con- 
vection.* The  subject  of  gaseous  conduction  has  been  re- 
cently taken  up  by  Professor  Magnus,  of  Berlin,  who  con- 
siders that  his  experiments  prove  that  hydrogen  gas  con- 
ducts heat  like  a  metal. 

The  cooling  action  of  air  may  be  thus  prettily  illustrat- 
ed— here  is  a  platinum  wire,  formed  into  a  coil ;  I  send  a 
voltaic  current  through  the  coil,  till  it  glows  bright  red.  I 
now  stretch  out  the  coil  so  as  to  form  a  straight  wire  ;  the 
glow  instantly  sinks — you  can  now  hardly  see  it.  This  effect 
.8  due  entirely  to  the  freer  access  of  the  cold  air  to  the 
stretched  wire.  Here,  again,  is  a  receiver  R  (fig.  72)  which 
can  be  exhausted  at  pleasure  ;  attached  to  the  bottom  is  a 
vertical  metal  rod,  m  ft,  and  through  the  top  another  rod, 
a  #,  passes,  which  can  be  moved  up  and  down  through  an 
air-tight  collar,  so  as  to  bring  the  ends  of  the  two  rods 
within  any  required  distance  of  each  other.  At  present 
the  rods  are  united  by  two  inches  of  platinum  wire,  b  m, 
which  I  can  heat  to  any  required  degree  of  intensity  by  a 
voltaic  current.  I  have  here  a  small  battery,  and  now  I 
make  my  connections  ;  the  wire  is  barely  luminous  enough 
to  be  seen ;  in  fact,  the  current  from  a  single  cell  only  is 
now  sent  through  it.  It  is  surrounded  by  air,  w^hich,  no 
doubt,  is  carrying  off  a  portion  of  its  heat.  I  exhaust 
the  receiver — the  wire  glows  more  brightly  than  before. 
I  allow  air  to  enter — the  wire,  for  a  time,  is  quite 
quenched,  rendered  perfectly  black ;  but  after  the  air  has 
ceased  to  enter,  its  first  feeble  glow  is  restored.  The  cur- 

*  Phil.  Trans.  1792:  Essays,  vol.  ii.  p.  56. 


258 


LECTURE   VII. 


rent  of  air  here  passing  over  the  wire,  and  destroying  its 
glow,  acts  like  the  current  which  the  wire  itself  establishes 
by  heating  the  air  in  contact  with  it. 
The  cooling  of  the  wire  in  both  cases  is 
due  to  convection  and  not  to  true  con- 
duction. 

The  same  effect  is  obtained  in  a  great- 
ly increased  degree,  if  hydrogen  be  used 
instead  of  air.  We  owe  this  interesting 
observation  to  Mr.  Grove,  and  it  formed 
the  starting-point  of  M.  Magnus's  investi- 
gation. The  receiver  is  now  exhausted, 
and  the  wire  is  almost  white-hot.  Air 
cannot  do  more  than  reduce  that  wrhitc- 
ness  to  bright  redness  ;  but  observe  what 
hydrogen  can  do.  On  the  entrance  of 
this  gas  the  wire  is  totally  quenched,  and 
even  after  the  receiver  has  been  filled 
with  the  gas,  and  the  inward  current  has 
ceased,  the  glow  of  the  wire  is  not  re- 
stored. The  electric  current  now  passing 
through  the  wire  is  from  two  cells  ;  I  try 
three  cells,  the  wire  glows  feebly ;  five  cause  it  to  glow 
more  brightly,  but  even  with  five  it  is  but  a  bright  red. 
Were  the  hydrogen  not  there,  the  current  now  passing 
through  the  wire  would  infallibly  fuse  it.  Let  us  see 
whether  this  is  not  the  case.  I  commence  exhaustion, — the 
first  few  strokes  of  the  pump  produce  a  scarcely  sensible 
effect;  but  I  continue  to  work  the  pump,  and  now  the 
effect  begins  to  be  visible.  The  wire  whitens  and  appears 
to  thicken.  To  those  at  a  distance  it  is  now  as  thick  as  a 
goose-quill ;  and  now  it  glows  upon  the  point  of  fusion  ;  I 
continue  to  work  the  pump,  the  light  suddenly  vanishes, 
the  wire  is  fused. 

This  extraordinary  cooling  power  of  hydrogen  has  been 


EXPERIMENTS   OF  MAGNUS.  259 

usually  ascribed  to  the  mobility  of  its  particles,  which  ena- 
bles currents  to  establish  themselves  in  this  gas  with  great- 
er facility  than  in  any  other.  But  Prof.  Magnus  conceives 
the  chilling  of  the  wire  to  be  an  effect  of  conduction.  To 
impede,  if  not  prevent,  the  formation  of  currents,  he  passes 
his  platinum  wire  along  the  axis  of  a  narrow  glass  tube, 
which  he  fills  with  hydrogen.  Although  in  this  case  the 
wire  is  surrounded  by  a  mere  film  of  the  gas,  and  currents, 
in  the  ordinary  sense,  are  Scarcely  to  be  assumed,  the  film 
shows  itself  just  as  competent  to  quench  the  wire,  as  when 
the  latter  is  caused  to  pass  through  a  large  vessel  contain- 
ing the  gas.  He  also  heated  the  closed  top  of  a  vessel,  and 
found  that  the  heat  was  conveyed  more  quickly  from  it  to 
a  thermometer,  placed  at  some  distance  below  the  source 
of  heat,  when  the  vessel  was  filled  with  hydrogen,  than 
when  it  was  filled  with  air.  He  found  this  to  be  the  case, 
even  when  the  vessel  was  loosely  filled  with  cotton  wool  or 
cider  down.  Here,  he  contends,  currents  could  not  be 
formed ;  the  heat  must  be  conveyed  to  the  thermometer  by 
the  true  process  of  conduction,  and  not  by  convection. 

Beautiful  and  ingenious  as  these  experiments  are,  I  do 
not  think  they  conclusively  establish  the  conductivity  of 
hydrogen.  Let  us  suppose  the  wire  in  Prof.  Magnus's  first 
experiment  to  be  stretched  along  the  axis  of  a  wide  cylin- 
der containing  hydrogen,  we  should  have  convection,  in  the 
ordinary  sense,  on  heating  the  wire.  Where  does  the  heat 
thus  dispersed  ultimately  go  ?  It  is  manifestly  given  up  to 
the  sides  of  the  cylinder,  and  if  we  narrow  our  cylinder  we 
simply  hasten  the  transfer.  The  process  of  narrowing  may 
continue  till  a  narrow  tube  is  the  result, — the  convection 
between  centre  and  sides  will  continue  and  produce  the 
same  cooling  effect  as  before.  The  heat  of  the  gas  being 
instantly  lowered  by  communication  to  the  heavy  tube,  it 
is  prepared  to  re-abstract  the  heat  from  the  wire.  With 
regard  also  to  the  vessel  heated  at  the  top,  it  would  require 


260  LECTTJKE  vn. 

a  surface  mathematically  horizontal,  and  a  perfectly  uniform 
application  of  heat  to  that  surface — it  would,  moreover,  be 
necessary  to  cut  the  heat  sharply  off  from  the  sides  of  the 
vessel — to  prevent  convection.  Even  in  the  interstices  of 
the  eider  down  and  of  the  cotton  wool  the  convective  mo- 
bility of  hydrogen  will  make  itself  felt,  and  taking  every- 
thing into  account,  I  think  the  experimental  question  of 
gaseous  conduction  is  still  an  open  one.* 

*  In  my  opinion,  the  question  of  liquid  conduction  also  demands  fur- 
tlier  investigation. 


LECTURE    VIII. 

[March  13,  1862.] 

COOLING  A  LOSS  OF  MOTION  :  TO  WHAT  IS  THIS  MOTION  IMPARTED  ? — EX- 
PERIMENTS ON  SOUND  BEARING  ON  THIS  QUESTION — EXPERIMENTS  ON 
LIGHT  BEARING  ON  THIS  QUESTION — THE  THEORIES  OF  EMISSION  AND 
UNDULATION — LENGTH  OF  WAVES  AND  NUMBER  OF  IMPULSES  OF  LIGHT 
— PHYSICAL  CAUSE  OF  COLOUR— INVISIBLE  RAYS  OF  THE  SPECTRUM 

THE  CALORIFIC  RAYS  BEYOND  THE  RED — THE  CHEMICAL  RAYS  BEYOND 
THE  BLUE — DEFINITION  OF  RADIANT  HEAT — REFLECTION  OF  RADIANT 
HEAT  FROM  PLANE  AND  CURVED  SURFACES  :  LAWS  THE  SAME  AS  THOSE 
OF  LIGHT — CONJUGATE  MIRRORS. 

APPENDIX: — ON  SINGING  FLAMES. 

WE  have  this  day  reached  the  boundary  of  one  of  the 
two  great  divisions  of  our  subject ;  hitherto  we 
have  dealt  with  heat  while  associated  with  solid,  liquid,  or 
gaseous  bodies.  ,  We  have  found  it  competent  to  produce 
changes  of  volume  in  all  these  bodies.  We  have  also  ob- 
served it  reducing  solids  to  liquids,  and  liquids  to  vapours  ; 
we  have  seen  it  transmitted  through  solids  by  the  process 
of  conduction,  and  distributing  itself  through  liquids  and 
gases  by  the  process  of  convection.  We  have  now  to  fol- 
low it  into  conditions  of  existence,  different  from  any 
which  we  have  examined  hitherto. 

I  hang  this  heated  copper  ball  in  the  air ;  you  see  it 
glow,  the  glow  sinks,  the  ball  becomes  obscure  ;  in  popular 
language  the  ball  cools.  Bearing  in  mind  what  has  been 
said  on  the  nature  of  heat,  we  must  regard  this  cooling  as 


262  LECTUKE  vm. 

a  loss  of  motion  on  the  part  of  the  ball.  But  motion  can- 
not be  lost  without  being  imparted  to  something ;  to  what 
then  is  the  molecular  motion  of  this  ball  transferred? 
You  would,  perhaps,  answer  to  the  air,  and  this  is  partly 
true :  over  the  ball  air  is  passing,  and  rising  in  a  heated 
column,  which  is  quite  visible  against  the  screen,  when  we 
allow  the  electric  beam  to  pass  through  the  warmed  air. 
But  not  the  whole,  nor  even  the  chief  part,  of  the  molecu- 
lar motion  of  the  ball  is  lost  in  this  way.  If  the  ball  were 
placed  in  vacuo  it  would  still  cool.  Rumford,  of  whom  we 
have  heard  so  much,  contrived  to  hang  a  small  thermom- 
eter, by  a  single  fibre  of  silk ',  in  the  middle  of  a  glass  globe 
exhausted  by  means  of  mercury,  and  he  found  that  the  cal- 
orific rays  passed  to  and  fro  across  the  vacuum  ;  thus  prov- 
ing that  the  transmission  of  the  heat  was  independent  of 
the  air.  Davy,  with  an  apparatus  which  I  have  here  be- 
fore me,  showed  that  the  heat  rays  from  the  electric  light 
passed  freely  through  an  air-pump  vacuum ;  and  we  can 
repeat  his  experiment  substantially  for  ourselves.  I  simply 
take  the  receiver  made  use  of  in  our  last  lecture  (fig.  72), 
and  removing  the  remains  of  the  platinum  wire,  then  de- 
stroyed, I  attach  to  each  end  of  the  two  rods,  m  n  and  a  &, 
a  bit  of  retort  carbon.  I  now  exhaust  the  receiver,  bring 
the  coal  points  together,  and  send  a  current  from  point  to 
point.  The  moment  I  draw  the  points  a  little  apart,  the 
electric  light  blazes  forth :  and  here  I  have  the  thermo- 
electric pile  ready  to  receive  a  portion  of  the  rays.  The 
galvanometer  needle  at  once  flies  aside,  and  this  has  been 
accomplished  by  rays  which  have  crossed  the  vacuum. 

But  if  not  to  the  air,  to  what  is  the  motion  of  our  cool- 
ing ball  communicated  ?  We  must  ascend  by  easy  stages 
to  the  answer  to  this  question.  It  was  a  very  considerable 
step  in  science  when  men  first  obtained  a  clear  conception 
of  the  way  hi  which  sound  is  transmitted-through  air,  and 
it  was  a  very  important  experiment  which  Hauksbee  made 


AND   SOUND.  263 

before  the  Royal  Society  in  1705,  by  which  he  showed  that 
sound  could  not  propagate  itself  through  a  vacuum.  Now 
I  wish  to  make  manifest  to  you  this  conveyance  of  the  vi- 
brations of  sound  through  the  air.  I  have  here  a  bell 
turned  up-side-down,  and  supported  by  a  stand.  I  draw  a 
fiddle-bow  across  the  edge  of  the  bell,  you  hear  its  tone ; 
the  bell  is  now  vibrating,  and  if  I  throw  sand  upon  its  flat- 
tish  bottom,  it  would  arrange  itself  there  so  as  to  form  a 
definite  figure,  or  if  I  filled  it  with  water  I  should  see  the 
surface  fretted  with  the  most  beautiful  crispations.  These 
crispations  would  show  that  the  bell,  when  it  emits  this 
note,  divides  itself  into  four  swinging  parts,  which  are  sep- 
arated from  each  other  by  lines  of  no  swinging.  Here  is 
a  sheet  of  tracing  paper,  drawn  tightly  over  this  hoop,  so 
as  to  form  a  kind  of  fragile  drum.  I  hold  it  over  the  vi- 
brating bell,  but  not  so  as  to  touch  the  latter  ;  you  hear  the 
shivering  of  the  membrane.  It  is  a  little  too  slack,  so  I 
will  tighten  it  by  warming  it  before  the  fire,  and  repeat 
the  experiment.  You  no  longer  hear  a  shivering,  but  a  loud 
musical  tone  superadded  to  that  of  the  bell.  I  raise  the 
membrane  and  lower  it ;  I  move  it  to  and  fro,  and  you  hear 
the  rising  and  the  sinking  of  the  tone.  Here  is  a  smaller 
drum,  which  I  pass  round  the  bell,  holding  the  membrane 
vertical ;  it  actually  bursts  into  a  roar  when  I  bring  it  within 
half  an  inch  of  the  bell.  The  motion  of  the  bell,  communi- 
cated to  the  air,  has  been  transmitted  by  it  to  the  mem- 
brane, and  the  latter  is  thus  converted  into  a  sonorous 
body. 

I  have  here  two  plates  of  brass,  A  B  (fig.  73),  united  to- 
gether by  this  metal  rod.  I  have  darkened  the  plates  by 
bronzing  them,  and  on  both  of  them  I  strew  a  quantity  of 
white  sand.  I  now  take  the  connecting  brass  rod  by  its 
centre,  between  the  finger  and  thumb  of  my  left  hand,  and 
holding  it  upright  I  draw,  with  my  right,  a  piece  of  flan- 
nel, over  which  I  have  shaken  a  little  powdered  resin,  along 


264 


LECTURE   VIII. 


the  rod.     You  hear  the  sound ;  but  observe  the  behaviour 

of  the  sand :    a  single 
Fis- 73-  stroke  of  my  finger,  you 

see,  has  caused  it  to 
jump  into  a  series  of 
concentric  rings,  which 
must  be  quite  visible  to 
you  all.  I  repeat  the 
experiment  operating 
more  gently;  you  hear 
the  clear,  weak,  musical 
sound,  you  see  the  sand 
shivering,  and  creeping, 
by  degrees,  to  the  lines 
which  it  formerly  occu- 
pied ;  and  there  are  the 
curves  as  sharply  drawn 
upon  the  surface  of  the 
lower  disk  as  if  they  had 
been  arranged  with  a 
camel's  hair  pencil.  On 
the  upper  disk  you  see  a 
series  of  concentric  cir- 
cles of  the  same  kind. 
In  fact,  the  vibrations 
which  I  have  imparted 
to  the  rod  have  commu- 
nicated themselves  to 
both  the  disks,  and  di- 
vided each  of  them  into 
a  series  of  vibrating  seg- 
ments, which  are  sepa- 
rated from  each  other 
by  lines  of  no  vibration,  on  which  the  sand  finds  peace. 
Now  let  me  show  you  the  transmission  of  these  vibra- 


COMMUNICATION   OF   VIBRATIONS   THROUGH   AIE.      265 

tions  from  the  lower  disk  through  the  air.  On  the  floor  I 
place  this  paper  drum,  D,  strewing  dark-coloured  sand  uni- 
formly over  it ;  I  might  stand  on  the  table — I  might  stand 
as  high  as  the  ceiling,  and  produce  the  effect  which  I  am 
now  going  to  show  you.  Pointing  the  rod  which  unites 
my  plates  in  the  direction  of  the  paper  drum,  I  draw  my 
rcsined  rubber  vigorously  over  the  rod :  observe  the  effect, 
— a  single  stroke  has  caused  that  sand  to  spring  into  a  reti- 
culated pattern.  A  precisely  similar  effect  is  produced  by 
sound  on  the  drum  of  the  ear ;  the  tympanic  membrane  is 
caused  to  shudder  in  the  same  manner  as  that  drum-head 
of  paper,  and  its  motion,  conveyed  to  the  auditory  nerves 
and  transmitted  thence  to  the  brain,  awakes  in  us  the  sen- 
sation of  sound. 

Here  is  a  still  more  striking  example  of  the  conveyance 
of  the  motion  of  sound  through  the  air.  By  permitting  a 
jet  of  gas  to  issue  through  the  small  orifice  of  this  tube,  I 
obtain  a  slender  flame,  and  by  turning  the  cock  I  reduce 
the  flame  to  a  height  of  about  half  an  inch.  I  introduce 
the  flame  into  this  glass  tube,  A  B  (fig.  74),  which  is  twelve 
inches  long.  Now  I  must  ask  your  permission  to  address 
that  flame,  and  if  I  am  skilful  enough  to  pitch  my  voice  to 
the  precise  note,  I  am  sure  the  flame  will  respond ;  it  will 
start  suddenly  into  a  melodious  song,  and  continue  singing 
as  long  as  the  gas  continues  to  burn.  The  burner  is  now 
arranged  within  the  tube,  which  covers  it  to  a  depth  of  a 
couple  of  inches.  If  I  were  to  lower  it  more,  the  flame 
would  start  into  singing  on  its  own  account,  as  in  the  well- 
known  case  of  the  hydrogen  harmonica ;  but,  with  the 
present  arrangement,  it  cannot  sing  till  I  tell  it  to  do  so. 
Now  I  emit  a  sound,  which  you  will  pardon  if  it  is  not 
musical.  The  flame  does  not  respond ;  I  have  not  spoken 
to  it  in  the  proper  language.  Let  me  try  again ;  I  pitch 
my  voice  a  little  higher ;  there,  the  flame  stretches  its  little 
throat,  and  every  individual  in  this  large  audience  hears 
12 


266 


LECTURE  Vin. 


Fig.  74. 


the  sound  of  it.    I  stop  the  song,  and  stand  at  a  greater 
distance  from  the  flame,  and  now  that  I  have  ascertained 

the  proper  pitch,  the  ex- 
periment is  sure  to  suc- 
ceed; from  a  distance  of 
twenty  or  thirty  feet  I  can 
cause  that  flame  to  sing.  I 
now  stop  it,  turn  my  back 
upon  it,  and  strike  the  note 
as  before ;  you  see  how 
obedient  it  is  to  my  voice  ; 
when  I  call,  it  answers,  and 
with  a  Mttle  practice  I  have 
been  able  to  command  the 
flame  to  sing  and  to  stop, 
and  it  has  strictly  obeyed 
the  injunction.  Here,  then, 
we  have  a  striking  example 
of  the  conveyance  of  the 
vibrations  of  the  organ  of 
voice  through  the  air,  and 
of  their  communication  to 
a  body  which  is  eminently 
sensitive  to  their  action.* 

Why  do  I  make  these 
experiments  on  sound  ? 
Simply  to  give  you  clear 

conceptions  regarding  what  takes  place  in  the  case  of  heat ; 
to  lead  you  up  from  the  tangible  to  the  intangible ;  from 
the  region  of  sense  into  that  of  physical  theory. 

After  philosophers  had  become  aware  of  the  manner  in 

*  Though  not  belonging  to  our  present  subject,  so  many  persons  have 
evinced  an  interest  in  this  experiment  that  I  have  been  induced  to  reprint 
two  short  papers  in  the  Appendix  to  this  Lecture,  in  which  the  experiment 
is  more  fully  described, 


TIIEOEIES   OF   EMISSION  AND  UNDULATION.  267 

which  sound  was  produced  and  transmitted,  analogy  led 
some  of  them  to  suppose  that  light  might  be  produced  and 
transmitted  in  a  somewhat  similar  manner.  An/l  perhaps 
in  the  whole  history  of  science  there  was  never  a  question 
more  hotly  contested  than  this  one.  Sir  Isaac  Newton  sup- 
posed light  to  consist  of  minute  particles  darted  out  from 
luminous  bodies  :  this  was  the  celebrated  Emission  Theory. 
Iluyghens,  the  contemporary  of  Newton,  found  great  diffi- 
culty in  conceiving  of  this  cannonade  of  particles ;  that 
they  should  shoot  with  inconceivable  velocity  through  space 
and  not  disturb  each  other.  This  celebrated  man  enter- 
tained the  view  that  light  was  produced  by  vibrations  sim- 
ilar to  those  of  sound.  Euler  supported  Huyghens,  and 
one  of  his  arguments,  though  not  quite  physical,  is  so 
quaint  and  curious  that  I  will  repeat  it  here.  He  looks  at 
our  various  senses,  and  at  the  manner  in  which  they  are 
affected  by  external  objects.  l  With  regard  to  smell,'  he 
says,  '  we  know  that  it  is  produced  by  material  particles 
which  issue  from  a  volatile  body.  In  the  case  of  hearing, 
nothing  is  detached  from  the  sounding  body,  and  in  the 
case  of  feeling  we  must  touch  the  body  itself.  The  dis- 
tance at  which  our  senses  perceive  bodies  is,  in  the  case  of 
touch,  no  distance,  in  the  case  of  smell  a  small  distance,  in 
the  case  of  hearing,  a  considerable  distance,  but  in  the  case 
of  sight  greatest  of  all.  It  is  therefore  more  probable  that 
the  same  mode  of  propagation  subsists  for  sound  and  light, 
than  that  odours  and  light  should  be  propagated  in  the 
same  manner  ; — that  luminous  bodies  should  behave,  not  as 
volatile  substances,  but  as  sounding  ones.' 

The  authority  of  Newton  bore  these  men  down,  and 
not  until  a  man  of  genius  within  these  walls  took  up  the 
subject,  had  the  Theory  of  Undulation  any  chance  of  co- 
ping with  the  rival  Theory  of  Emission.  To.  Dr.  Thomas 
Young,  who  was  formerly  Professor  of  Natural  Philos- 
ophy in  this  Institution,  belongs  the  immortal  honour  of 


268  LECTURE  vm. 

stemming  this  tide  of  authority,  and  of  establishing  on  a 
safe  basis,  the  theory  of  undulation.  There  have  been  great 
things  done  in  this  edifice,  but  hardly  a  greater  than  this. 
And  Young  was  led  to  his  conclusion  regarding  light,  by  a 
series  of  investigations  on  sound.  He,  like  ourselves,  at 
the  present  moment,  rose  from  the  known  to  the  unknown, 
from  the  tangible  to  the  intangible.  This  subject  has  been 
illustrated  and  enriched  by  the  labours  of  genius  ever  since 
the  time  of  Young ;  but  one  name  only  will  I  here  asso- 
ciate with  his, — a  name  which,  in  connection  with  this  sub- 
ject, can  never  be  forgotten :  that  is,  the  name  of  Augustin 
Fresnel. 

According  to  the  notion  now  universally  received,  light 
consists,  first,  of  a  vibratory  motion  of  the  particles  of  the 
luminous  body  ;  but  how  is  this  motion  transmitted  to  our 
organs  of  sight  ?  Sound  has  the  air  as  its  medium,  and 
long  pondering  on  the  phenomena  of  light,  and  refined 
and  conclusive  experiments,  devised  with  the  express  inten- 
tion of  testing  the  idea,  have  led  philosophers  to  the  con- 
clusion, that  space  is  occupied  by  a  substance  almost  in- 
finitely elastic,  through  which  the  pulses  of  light  make 
their  way.  Here  your  conceptions  must  be  perfectly  clear. 
The  intellect  knows  no  difference  between  great  and  small : 
it  is  just  as  easy,  as  an  intellectual  act,  to  conceive  of  a  vi- 
brating atom  as  to  conceive  of  a  vibrating  cannon-ball ;  and 
there  is  no  more  difficulty  in  conceiving  of  this  Ether,  as  it 
is  called,  which  fills  space,  that  in  imagining  all  space  to  be 
filled  with  jelly.  You  must  imagine  the  atoms  vibrating, 
and  their  vibrations  you  must  figure  as  communicated  to 
the  ether  in  which  they  swing,  being  propagated  through  it 
in  waves  ;  these  waves  enter  the  pupil,  cross  the  ball  of  the 
eye,  and  break  upon  the  retina  at  the  back  of  the  eye.  The 
act,  remember,  is  as  real,  and  as  truly  mechanical  as  the 
breaking  of  the  sea  waves  upon  the  shore.  Their  motions 
are  communicated  to  the  retina,  transmitted  thence  along 


INTERSTELLAR   MEDIUM.  269 

the  optic  nerve  to  the  brain,  and  there  announce  them- 
selves to  consciousness  as  light. 

I  have  here  an  electric  lamp,  known  well  to  all  of  you, 
and  on  the  screen  in  front  of  you  I  project  an  image  of  the 
incandescent  coal  points  which  produce  the  electric  light. 
I  will  first  bring  the  points  together  and  then  separate  them. 
Observe  the  effect.  You  have  first  the  place  of  contact  ren- 
dered luminous,  then  you  see  the  glow  conducted  downwards 
to  a  certain  distance  along  the  stem  of  coal.  This,  as  you 
know,  is  in  reality  the  conduction  of  motion.  I  interrupt 
the  circuit.  The  points  continue  to  glow  for  a  short  time  ; 
the  light  is  now  subsiding.  The  coal  points  are  now  quite 
dark,  but  have  they  ceased  to  radiate  ?  By  no  means.  At 
the  present  moment  there  is  a  copious  radiation  from  these 
points,  which,  though  incompetent  to  affect  sensibly  the 
nerves  of  vision,  are  quite  competent  to  affect  other  nerves 
of  the  human  system.  To  the  eye  of  the  philosopher  who 
looks  at  such  matters  without  reference  to  sensation,  these 
obscure  radiations  are  precisely  the  same  in  kind  as  those 
which  produce  the  impression  of  light.  You  must  there- 
fore figure  the  particles  of  the  heated  body  ;as  being  in  a 
state  of  motion ;  you  must  figure  the  motion  communicated 
to  the  surrounding  ether,  and  transmitted  through  the  ether 
with  a  velocity,  which  we  have  the  strongest  reason  for  be- 
lieving is  the  same  as  that  of  light.  Thus  when  you  turn 
towards  a  fire  on  a  cold  day,  and  expose  your  chilled  hands 
to  its  influence,  the  warmth  that  you  feel  is  due  to  the  im- 
pact of  these  ethereal  billows  upon  your  skin  ;  they  throw 
the  nerves  into  motion,  and  the  consciousness  correspond- 
ing to  this  motion  is  what  we  popularly  call  warmth.  Our 
task  during  the  lectures  which  remain  to  us  is  to  examine 
heat  under  this  radiant  form. 

To  investigate  this  subject  we  possess  our  valuable  ther- 
mo-electric pile,  the  face  of  which  is  now  coated  with  lamp- 
black, a  powerful  absorber  of  radiant  heat.  I  hold  the  in- 


270  LECTURE   VIII. 

strument  in  front  of  the  cheek  of  Mr.  Anderson ;  he  is  a  ra- 
diant body,  and  observe  the  effect  produced  by  his  rays ; 
the  pile  drinks  them  in,  they  generate  electricity,  and  the 
needle  of  the  galvanometer  moves  up  to  90°.  I  withdraw 
the  pile  from  the  source  of  heat,  and  allow  the  needle  to 
come  to  rest,  and  now  I  place  this  slab  of  ice  in  front  of 
the  pile.  You  have  a  deflection  in  the  opposite  direction, 
as  if  rays  of  cold  were  striking  on  the  pile.  But  you  know 
that  in  this  case  the  pile  is  the  hot  body ;  it  radiates  its 
heat  against  the  ice ;  the  face  of  the  pile  is  thus  chilled, 
and  the  needle,  as  you  see,  moves  up  to  90°  on  the  side  of 
cold.  Our  pile  is  therefore  not  only .  available  for  the 
examination  of  heat  communicated  to  it  by  direct  contact, 
but  also  for  the  examination  of  radiant  heat.  Let  us  ap- 
ply it  at  once  to  a  most  important  investigation,  and  exam- 
ine, by  means  of  it,  the  distribution  of  thermal  power  in 
the  electric  spectrum. 

Let  me  in  the  first  place  show  you  this  spectrum.  I  do 
so  by  sending  a  slice  of  pure  white  light  from  the  orifice 
o  (fig.  75),  through  this  prism,  a  f>  c,  which  is  built  up  of 

Fig.  75. 


plane  glass  sides,  but  is  filled  with  the  liquid  bisulphide  of 
carbon.  It  gives  a  richer  display  of  colour  than  glass  does, 
and  this  is  one  reason  why  I  use  it  in  preference  to  glass. 
Here  then  you  have  the  white  beam  disentangled,  and  re- 


HEAT  OF  SPECTEmi. 


271 


duccd  to  the  colours  which  compose  it ;  you  have  this  burn- 
ing red,  this  vivid  orange,  this  dazzling  yellow,  this  brill- 
iant green,  and  these  various  shades  of  blue  ;  the  blue  space 
being  usually  subdivided  into  blue,  indigo,  and  violet.  I 
will  now  cause  a  thermo-electric  pile  of  particular  construc- 
tion to  pass  gradually  through  all  these  colours  in  succes- 
sion, so  as  to.  test  their  heating  powers,  and  I  will  ask  you 
to  observe  the  needle  of  the  galvanometer  which  is  to  de- 
clare the  magnitude  of  that  power. 

For  this  purpose  I  have  here  (fig.  76)  a  beautiful  piece 
of  apparatus,  designed  by  Melloni,  and  executed,  with  his 
accustomed  skill,  by  M.  Ruhm- 
korff.*  You  observe  here  a  pol- 
ished brass  plate,  A  B,  attached 
to  a  stem,  and  this  stem  is 
mounted  on  a  horizontal  bar, 
which,  by  means  of  a  screw,  has 
motion  imparted  to  it.  By  turn- 
ing this  ivory  handle  in  one  di- 
rection I  cause  the  plate  of  brass 
to  approach ;  by  turning  it  in  the 
other,  I  cause  it  to  recede,  and 
the  motion  is  so  fine  and  gradual, 
that  I  could,  with  ease  and  cer- 
tainty, push  the  screen  through  a 
space  less  than  2  oVwth  of  an  inch. 
You  observe  a  narrow  vertical 
slit  in  the  middle  of  this  plate, 
and  something  dark  behind  it. 
That  dark  space  is  the  blackened  face  of  a  thermo-electric 
pile,  P,  the  elements  of  which  are  ranged  in  a  single  row, 
and  not  in  a  square,  as  in  our  other  instrument.  I  will  al- 
low distinct  slices  of  the  spectrum  to  fall  on  that  slit ;  each 
will  impart  whatever  heat  it  possesses  to  the  pile,  and  the 

*  Kindly  lent  to  me  by  M.  Gassiot. 


272  LECTURE  VIII. 

quantity  of  the  heat  will  be  marked  by  the  needle  of  our 
galvanometer. 

At  present  a  small  but  brilliant  spectrum  falls  upon  the 
plate,  A  B,  but  the  slit  is  quite  out  of  the  spectrum.  I  turn 
the  handle,  and  the  slit  gradually  approaches  the  violet 
end  of  the  spectrum ;  the  violet  light  now  falls  upon  the 
slit,  but  the  needle  does  not  move  sensibly.  I  pass  on  to 
the  indigo,  the  needle  is  still  quiescent;  the  blue  also 
shows  no  action.  I  pass  on  to  the  green,  the  needle  bare- 
ly stirs :  now  the  yellow  falls  upon  the  slit ;  the  motion  of 
the  needle  is  now  perhaps  for  the  first  time  visible  to  you ; 
but  the  deflection  is  small,  though  I  now  expose  the  pile 
to  the  most  luminous  part  of  the  spectrum.*  I  will  now 
pass  on  to  the  orange,  which  is  less  luminous  than  the  yel- 
low, but  you  observe,  though  the  light  diminishes  the  heat 
increases ;  the  needle  moves  still  farther.  I  pass  on  to  the 
red,  which  is  still  less  luminous  than  the  orange,  and  you 
see  that  I  here  obtain  the  greatest  thermal  power  exhibited 
by  any  of  the  visible  portions  of  the  spectrum. 

The  appearance,  however,  of  this  burning  red  might  lead 
you  to  suppose  it  natural  for  such  a  colour  to  be  hotter 
than  any  of  the  others.  But  now  pay  attention.  I  will 
cause  my  slit  to  pass  entirely  out  of  the  spectrum,  quite 
beyond  the  extreme  red.  Look  to  the  galvanometer !  The 
needle  goes  promptly  up  to  the  stops.  So  that  we  have 
here  a  heat-spectrum  wrhich  we  cannot  see,  and  whose  ther- 
mal power  is  far  greater  than  that  of  any  visible  part  of 
the  spectrum.  In  fact,  the  electric  light  with  which  we 
deal,  emits  an  infinity  of  rays  which  are  converged  by  our 
lens,  refracted  by  our  prism,  which  form  the  prolongation 
of  our  spectrum,  but  which  are  utterly  incompetent  to  ex- 
cite the  optic  nerve  to  vision.  It  is  the  same  with  the  sun. 
Our  orb  is  rich  in  these  obscure  rays  ;  and  though  they  are 

*  I  am  here  dealing  with  a  large  lecture-room  galvanometer. 


EXTKA  EED  AND  EXTRA  VIOLET  KAYS.       273 

for  the  most  part  cut  off  by  our  atmosphere,  multitudes  of 
them  still  reach  us.  To  the  great  William  Herschel  we  are 
indebted  for  the  discovery  of  them. 

Thus  we  prove  that  the  spectrum  extends  on  the  red 
side  much  beyond  its  visible  limits ;  and  were  I,  instead  of 
being  compelled  to  make  use  of  lenses  and  prisms  of  glass, 
fortunate  enough  to  possess  lenses  and  prisms  of  rock  salt,  I 
could  show  you,  as  Melloni  has  done,  that  those  rays  extend 
a  great  way  farther  than  it  is  now  in  my  power  to  prove. 
In  fact,  glass,  though  sensibly  transparent  to  light,  is,  in  a 
great  measure,  opaque  to  these  obscure  rays ;  instead  of 
reaching  the  screen,  they  are  for  the  most  part  lodged  in 
the  glass. 

The  visible  spectrum,  then,  simply  marks  an  interval  of 
radiant  action,  in  which  the  radiations  are  so  related  to  our 
organisation  that  they  excite  the  impression  of  light ;  be- 
yond this  interval,  in  both  directions,  radiant  power  is 
exerted — obscure  rays  fall — those  falling  beyond  the  red 
being  powerful  to  produce  heat,  while  those  falling  beyond 
the  violet  are  powerful  to  promote  chemical  action.  These 
latter  rays  can  actually  be  rendered  visible  ;  or  more  strict- 
ly expressed,  the  undulations  or  waves  which  are  now 
striking  here  beyond  the  violet  against  the  screen,  and 
which  are  scattered  from  it  so  as  to  strike  the  eyes  of  every 
person  present,  though  they  are  incompetent  to  excite 
vision  in  those  eyes  ;  those  waves,  I  say,  may  be  caused  to 
impinge  upon  another  body,  and  to  impart  their  motion  to 
it,  and  actually  to  convert  the  dark  space  beyond  the  violet 
into  a  brilliantly  illuminated  one.  I  have  here  the  proper 
substance.  The  lower  half  of  this  sheet  of  paper  has  been 
washed  with  a  solution  of  sulphate  of  quinine,  while  I  have 
left  the  upper  half  in  its  natural  state.  I  will  hold  the 
sheet,  so  that  the  straight  line  dividing  its  prepared  from 
its  unprepared  half,  shall  be  horizontal  and  shall  cut  the 
spectrum  into  two  equal  parts  ;  the  upper  half  will  remain 


274  LECTURE  vm. 

unaltered,  and  you  will  be  able  to  compare  with  it  the 
under  half,  on  which  I  hope  to  find  the  spectrum  elongated. 
You  see  this  effect ;  we  have  here  a  splendid  fluorescent 
band,  several  inches  in  width,  where  a  moment  ago  there 
was  nothing  but  darkness.  I  remove  the  prepared  paper, 
and  the  light  disappears.  I  re-introduce  it,  and  the  light 
flashes  out  again,  showing  you,  in  the  most  emphatic  man- 
ner, that  the  visible  limits  of  the  ordinary  spectrum  by  no 
means  mark  the  limits  of  radiant  action.  I  dip  my  brush 
in  this  solution  of  sulphate  of  quinine,  and  dab  it  against 
the  paper ;  wherever  the  solution  falls,  light  flashes  forth. 
The  existence  of  these  extra  violet  rays  has  been  long 
known ;  it  was  known  to  Thomas  Young,  who  actually  ex- 
perimented on  them  ;  but  to  Prof.  Stokes  we  are  indebted 
for  the  complete  investigation  of  this  subject.  lie  rendered 
the  rays  thus  visible. 

How  then  are  WTC  to  conceive  of  the  rays,  visible  and 
invisible,  which  fill  this  large  space  upon  the  screen  ?  Why 
are  some  of  them  visible  and  others  not  ?  Why  are  the 
visible  ones  distinguished  by  various  colours  ?  Is  there 
anything  that  we  can  lay  hold  of  in  the  undulations  which 
produce  these  colours,  to  which,  as  a  physical  cause,  we 
must  assign  the  colour  ?  Observe  first,  that  the  entire 
beam  of  white  light  is  drawn  aside,  or  refracted  by  the 
prism,  but  the  violet  is  pulled  aside  more  than  the  indigo, 
the  indigo  more  than  the  blue,  the  blue  more  than  the 
green,  the  green  more  than  the  yellow,  the  yellow  more 
than  the  orange,  and  the  orange  more  than  the  red.  These 
colours  are  differently  refrangible,  and  upon  this  depends 
the  possibility  of  their  separation.  To  every  particular  de- 
gree of  refraction  belongs  a  definite  colour  and  no  other. 
But  why  should  light  of  one  degree  of  refrangibility  pro- 
duce the  sensation  of  red,  and  of  another  degree  the  sensa- 
tion of  green  ?  This  leads  us  to  consider  more  closely  the 
cause  of  these  sensations. 


PHYSICAL   CAUSE   OF   COLOUR.  275 

A  reference  to  the  phenomena  of  sound  will  materially 
help  our  conceptions  here.  Figure  clearly  to  your  minds  a 
harp-string  vibrating  to  and  fro  ;  it  advances  and  causes  the 
particles  of  air  in  front  of  it  to  crowd  together ;  it  thus 
produces  a  condensation  of  the  air.  It  retreats,  and  the 
air  particles  behind  it  separate  more  widely;  in  other 
words,  a  rarefaction  of  the  air  occurs  behind  the  retreating 
wire.  The  string  again  advances  and  produces  the  conden- 
sation as  before,  it  again  retreats  and  produces  a  rarefac- 
tion. Thus  the  condition  of  the  air  through  which  the 
sound  of  the  string  is  propagated  consists  of  a  regular 
sequence  of  condensations  and  rarefactions,  which  travel 
with  a  velocity  of  about  1,100  feet  a  second. 

The  condensation  and  rarefaction  constitute  what  is 
called  a  sonorous  pulse  or  wave,  and  the  length  of  the  wave 
is  the  distance  from  the  middle  of  the  condensation  to  the 
middle  of  the  rarefaction.  Of  course  these  blend  gradually 
into  each  other.  The  length  of  the  wave  is  also  measured 
by  the  distance  from  the  centre  of  one  condensation  to  the 
centre  of  the  next  one.  Now  the  quicker  a  string  vibrates 
the  more  quickly  will  these  pulses  follow  each  other,  and 
the  shorter,  at  the  same  time,  will  be  the  length  of  each  in- 
dividual wave.  Upon  these  differences  the  pitch  of  a  note 
in  music  depends.  If  a  violin  player  wishes  to  produce  a 
higher  note,  he  shortens  his  string  by  pressing  his  finger 
on  it ;  he  thereby  augments  the  rapidity  of  vibration.  If 
his  point  of  pressure  exactly  halves  the  length  of  his  string, 
he  obtains  the  octave  of  the  note  which  the  string  emits 
when  vibrating  as  a  whole.  '  Boys  are  chosen  as  choristers 
to  produce  the  shrill  notes,  men  to  produce  the  bass  notes  ; 
the  reason  being,  that  the  boy's  organ  vibrates  more  speed- 
ily than  the  man's  ; '  and  the  hum  of  a  gnat  is  shriller  than 
that  of  a  beetle,  because  the  smaller  insect  can  send  a 
greater  number  of  impulses  per  second  to  the  ear. 

We  have  now  cleared  our  way  towards  the  clear  com- 


276  LECTUBE  vm. 

prehension  of  the  physical  cause  of  colour.  This  spectrum 
is  to  the  eye  what  the  gamut  is  to  the  ear ;  its  different 
colours  represent  notes  of  different  pitch.  The  vibrations 
which  produce  the  impression  of  red  are  slower,  and  the 
ethereal  waves  which  they  generate  are  longer,  than  those 
which  produce  the  impression  of  violet,  while  the  other 
colours  are  excited  by  waves  of  some  intermediate  length. 
The  length  of  the  waves  both  of  sound  and  light,  and  the 
number  of  shocks  which  they  respectively  impart  to  the  ear 
and  eye,  have  been  strictly  determined.  Let  us  here  go 
through  a  simple  calculation.  Light  travels  through  space 
at  a  velocity  of  192,000  miles  a  second.  Reducing  this  to 
inches,  we  find  the  number  to  be  12,165,120,000.  Now  it 
is  found  that  39,000  waves  of  red  light  placed  end  to  end 
would  make  up  an  inch ;  multiply  the  number  of  inches  in 
192,000  miles  by  39,000,  we  obtain  the  number  of  waves 
of  red  light  in  192,000  miles  :  this  number  is  474,439,680,- 
000,000.  All  these  waves  enter  the  eye  in  a  single  second. 
To  produce  the  impression  of  red  in  the  brain,  the  retina 
must  be  hit  at  this  almost  incredible  rate.  To  produce  the 
impression  of  violet,  a  still  greater  number  of  impulses  is 
necessary  ;  it  would  take  57,500  waves  of  violet  to  fill  an 
inch,  and  the  number  of  shocks  required  to  produce  the 
impression  of  this  colour,  amounts  to  six  hundred  and  nine- 
ty-nine millions  of  millions  per  second.  The  other  colours 
of  the  spectrum,  as  already  stated,  rise  gradually  in  pitch 
from  the  red  to  the  violet. 

But  beyond  the  violet  we  have  rays  of  too  high  a  pitch 
to  be  visible,  and  beyond  the  red  we  have  rays  of  too  low 
a  pitch  to  be  visible.  The  phenomena  of  light  are  in  this 
case  also  paralleled  by  those  of  sound.  If  it  did  not  in- 
volve a  contradiction,  we  might  say  that  there  are  musical 
sounds  of  too  high  a  pitch  to  be  heard,  and  also  sounds  of 
too  low  a  pitch  to  be  heard.  Speaking  strictly,  there  are 
waves  transmitted  through  the  air  from  vibrating  bodies, 


THEORY   OF   EXCHANGES.  277 

which,  though  they  strike  upon  the  air  in  regular  recur- 
rence, are  incompetent  to  excite  the  sensation  of  a  musical 
note.  Probably  sounds  are  heard  by  insects  which  entirely 
escape  our  perceptions  ;  and,  indeed,  as  regards  human  be- 
ings, the  selfsame  note  may  be  of  piercing  shrillness  to  one 
person,  while  it  is  absolutely  unheard  by  another.  Both 
as  regards  light  and  sound,  our  organs  of  sight  and  hearing 
embrace  a  certain  practical  range,  beyond  which,  on  both 
sides,  though  the  objective  cause  exists,  our  nerves  cease  to 
be  influenced  by  it. 

When  therefore  I  place  this  red-hot  copper  ball  before 
you,  and  watch  the  waning  of  its  light,  you  will  have  a 
perfectly  clear  conception  of  what  is  occurring  here.  The 
atoms  of  the  ball  oscillate,  but  they  oscillate  in  a  .resisting 
medium  on  which  their  moving  force  is  expended,  and 
which  transmits  it  on  all  sides  with  inconceivable  velocity. 
The  oscillations  competent  to  produce  light  are  now  ex- 
hausted ;  the  ball  is  quite  dark,  still  its  atoms  oscillate,  and 
still  their  oscillations  are  taken  up  and  transmitted  on  all 
sides  by  the  ether.  The  ball  cools  as  it  thus  loses  its 
molecular  motion,  but  no  cooling  to  which  it  can  be  prac- 
tically subjected  can  entirely  deprive  it  of  its  motion.  That 
is  to  say,  all  bodies,  whatever  may  be  their  temperature, 
are  radiating  heat.  From  the  body  of  every  individual 
here  present,  waves  are  speeding  away,  some  of  which 
strike  upon  this  cooling  ball  and  restore  a  portion  of  its 
lost  motion.  But  the  motion  thus  received  by  the  ball  is 
far  less  than  what  it  communicates,  and  the  difference  be- 
tween them  expresses  the  ball's  loss  of  motion.  As  long 
as  this  state  of  things  continues  the  ball  will  continue  to 
show  an  ever-lowering  temperature :  its  temperature  will 
sink  until  the  quantity  it  emits  is  equal  to  the  quantity 
which  it  receives,  and  at  this  point  its  temperature  becomes 
constant.  Thus,  though  you  are  conscious  of  no  reception 
of  heat,  when  you  stand  before  a  body  of  your  own  tern- 


278  LECTURE   VIII. 

perature,  an  interchange  of  rays  is  passing  between  you. 
Every  superficial  atom  of  each  mass  is  sending  forth  its 
waves,  which  cross  those  that  move  in  the  opposite  direc- 
tion, every  wave  asserting  its  own  individuality  amid  the 
entanglement  of  its  fellows.  When  the  sum  of  motion  re- 
ceived is  greater  than  that  given  out,  warming  is  the  con- 
sequence ;  when  the  sum  of  motion  given  out  is  greater 
than  that  received,  chilling  takes  place.  This  is  Prevost's 
Theory  of  Exchanges,  expressed  in  the  language  of  the 
Wave  Theory. 

Let  us  occupy  the  remainder  of  this  lecture  by  illustrat- 
ing experimentally  the  analogy  between  light  and  radiant 
heat,  as  regards  reflection.  You  observed  when  I  placed 
my  thermo-electric  pile  in  front  of  Mr.  Anderson's  face,  that 
I  had  attached  to  it  an  open  cone  which  I  did  not  use  in 
my  former  experiments.  This  cone  is  silvered  inside,  and 
it  is  intended  to  augment  the  action  of  feeble  radiations, 
by  converging  them  upon  the  face  of  the  thermo-electric 
pile.  It  does  this  by  reflection  ;  instead  of  shooting  wide 
of  the  pile,  as  they  would  do  if  the  reflector  were  removed, 
they  meet  the  silvered  surface  and  glance  from  it  against  the 
pile.  The  augmentation  of  the  effect  is  thus  shown.  I  place 
the  pile  at  this  end  of  the  table  with  its  reflector  off,  and  at 
a  distance  of  four  or  five  feet  I  place  this  copper  ball,  hot 
— but  not  red-hot ;  you  observe  scarcely  any  motion  of  the 
needle  of  the  galvanometer.  Disturbing  nothing,  I  now  at- 
tach the  reflector  to  the  pile ;  the  needle  instantly  goes  up 
to  90°,  declaring  the  augmented  action. 

The  law  of  this  reflection  is  precisely  the  same  as  that 
of  light.  Observe  this  apparently  solid  luminous  cylinder, 
issuing  from  our  electric  lamp,  and  marking  its  track  thus 
vividly  upon  the  dust  of  our  darkened  room.  I  take  a  mir- 
ror in  my  hand,  and  permit  the  beam  to  fall  upon  it ;  the 
beam  rebounds  from  the  mirror ;  it  now  strikes  the  ceiling. 
This  horizontal  beam  is  the  incident  beam,  this  vertical 


REFLECTION  FROM  PLANE  SURFACES.       279 

one  is  the  reflected  beam,  and  the  law  of  light,  as  many  of 
you  know,  is,  that  the  angle  of  incidence  is  equal  to  the 
angle  of  reflection.  The  incident  and  reflected  beams  now 
enclose  a  right  angle,  and  when  this  is  the  case  I  may  be 

Fig.  77. 


sure  that  both  beams  form,  with  a  perpendicular  to  the 
surface  of  the  mirror,  an  angle  of  45°. 

I  place  the  lamp  at  this  corner,  E,  of  the  table  (fig.  77)  ; 
behind  the  table  I  place  a  looking-glass,  L,  and  on  the  table 
you  observe  I  have  drawn  a  large  arc,  a  b.  Attached  to 
the  mirror  is  this  long  straight  lath,  m  n,  and  the  looking- 
glass,  resting  upon  rollers,  can  be  turned  by  the  lath,  which 
is  to  serve  as  an  index.  I  have  here  drawn  a  dark  central 
line,  and  when  the  mirror  exactly  faces  the  middle  of  the 
audience,  our  lath  and  this  line  coincide.  Those  in  front 
may  see  that  the  lath  itself  and  its  reflection  in  the  mirror 
form  a  straight  line,  which  proves  that  the  central  dark 
line  is  now  perpendicular  to  the  mirror.  Right  and  left  of 
this  central  line  I  have  divided  the  arc  into  ten  equal 
parts  ;  commencing  at  the  end  E  with  0°,  I  have  graduated 
the  arc  up  to  20°.  I  first  turn  the  index  so  that  it  shall  be 
in  the  line  of  the  beam  emitted  by  the  lamp.  The  beam 
now  falls  upon  the  mirror,  striking  it  as  a  perpendicular, 
and  you  see  it  is  reflected  back  along  the  line  of  incidence. 
I  now  move  my  index  to  1 ;  the  reflected  beam,  as  you  ob- 


280  LECTURE  VIII. 

serve,  draws  itself  along  the  table,  cutting  the  figure  2.  I 
move  the-  index  to  2,  the  beam  is  now  at  4  ;  I  move  the  in- 
dex to  3,  the  beam  is  now  at  6  ;  I  move  it  to  5,  the  beam 
is  now  at  10 ;  I  move  it  to  10,  the  beam  is  now  at  20.  If  I 
stand  midway  between  the  incident  and  reflected  beams, 
and  stretch  out  my  arms,  my  finger  tips  touch  each  of  them. 
One  lies  as  much  to  the  left  of  the  perpendicular  as  the 
other  does  to  the  right.  The  angle  of  incidence  is  equal  to 
the  angle  of  reflection.  But  we  have  also  demonstrated 
that  the  beam  moves  twice  as  fast  as  the  index  ;  and  this 
is  usually  expressed  in  the  statement,  that  the  angular  ve- 
locity of  a  reflected  ray  is  twice  that  of  the  mirror  which 
reflects  it. 

I  have  already  shown  you  that  these  incandescent  coal- 
points  emit  an  abundance  of  obscure  rays — of  rays  of  pure 
heat,  which  have  no  illuminating  power ;  my  object  now  is 
to  show  you  that  those  rays  of  heat  emitted  by  the  lamp, 
have  obeyed  precisely  the  same  laws  as  the  rays  of  light. 
I  have  here  a  piece  of  black  glass  ;  so  black  that  when  I 
look  through  it  at  the  electric  light,  or  even  at  the  noonday 
sun,  I  see  nothing.  You  observe  the  disappearance  of  the 
beam  when  I  place  this  glass  in  front  of  the  lamp.  It  cuts 
off  every  ray  of  light ;  but,  strange  as  it  may  appear  to 
you,  it  is,  in  a  considerable  degree,  transparent  to  the  ob- 
scure rays  of  the  lamp.  I  now  extinguish  the  light  by  in- 
terrupting the  current,  and  I  lay  my  thermo-electric  pile  on 
the  table  at  the  number  20,  where  the  luminous  beam  fell  a 
moment  ago.  The  pile  is  connected  with  the  galvanometer, 
and  the  needle  of  the  instrument  is  now  at  zero.  I  ignite 
the  lamp,  no  light  makes  its  appearance,  but  observe  the 
galvanometer ;  the  needle  has  already  swung  to  90°, 
through  the  action  of  the  non-luminous  rays  upon  the  pile. 
If  I  move  the  instrument  right  or  left  from  its  present  po- 
sition the  needle  immediately  sinks  ;  the  calorific  rays  have 
pursued  the  precise  track  of  the  luminous  rays ;  and  for 


RADIANT  HEAT  AND  LIGHT  OBEY  THE  SAME  LAW.   281 

them,  also,  the  angle  of  incidence  is  equal  to  the  angle  of 
reflection.  Repeating  the  experiments  that  I  have  already 
executed  with  light,  bringing  the  index  in  succession  to  1, 
2,  3,  5,  &c.,  I  prove  that  in  the  case  of  radiant  heat  also, 
the  angular  velocity  of  the  reflected  ray  is  twice  that  of  the 
mirror. 

The  heat  of  the  fire  obeys  the  same  law.  I  have  here  a 
sheet  of  tin — a  homely  reflector,  but  it  will  answer  my  pur- 
pose. At  this  end  of  the  table  I  place  the  thermo-electric 
pile,  and  at  the  other  end  my  tin  screen.  The  needle  of 
the  galvanometer  is  now  at  zero.  Well,  I  turn  the  reflector 
so  as  to  cause  the  heat  striking  it  to  rebound  towards  the 
pile  ;  it  now  meets  the  instrument,  and  the  needle  at  once 
declares  its  arrival.  Observe  the-  positions  of  the  fire,  of 
the  reflector,  and  of  the  pile  ;  you  see  that  they  are  just  in 
the  positions  which  make  the  angle  of  incidence  equal  to 
that  of  reflection. 

But  in  these  experiments  the  heat  is,  or  has  been,  asso- 
ciated with  light.  Let  me  now  show  that  the  law  holds 
good  for  rays  emanating  from  a  truly  obscure  body.  Here 
is  a  copper  ball,  c  (fig.  78),  heated  to  dull  redness ;  I  plunge 
it  in  water  until  its  light  totally  disappears,  but  I  leave  it 
warm.  It  is  still  giving  out  radiant  heat  of  a  slightly 
greater  intensity  than  that  emitted  by  the  human  body.  1 
place  it  on  this  candlestick  as  a  support,  and  here  I  place 
my  pile,  P,  turning  its  conical  reflector  away  from  the  ball,  so 
that  no  direct  ray  from  the  latter  can  reach  the  pile.  You 
see  the  needle  remains  at  zero.  I  place  here  my  tin  reflect- 
or, M  N,  so  that  a  line  drawn  to  it  from  the  ball,  shall  make 
the  same  angle  with  a  perpendicular  to  the  polished  tin  re- 
flector, as  a  line  drawn  from  the  pile.  The  axis  of  the  con- 
ical reflector  lies  in  this  latter  line.  True  to  the  law,  the 
heat-rays  emanating  from  the  ball  rebound  from  it  and 
strike  the  pile,  and  you  observe  the  consequent  prompt  mo- 
tion of  the  needle. 


282 


LECTURE 


Like  the  rays  of  light,  the  rays  of  heat  emanating  from 
our  ball  proceed  in  straight  lines  through  space,  diminish- 


ng. 73. 


ing  in  intensity  exactly  as  light  diminishes.  Thus,  this  ball, 
which  when  close  to  the  pile  causes  the  needle  of  the  gal- 
vanometer to  fly  up  to  90°,  at  a  distance  of  4  feet  6  inches, 
shows  scarcely  a  sensible  action.  Its  rays  are  squandered  on 
all  sides,  and  comparatively  few  of  them  reach  the  pile.  But 
I  now  introduce  between  the  pile  and  the  ball  this  tin  tube, 
A  B  (fig.  79),  4  feet  long.  It  is  polished  within,  and  there- 
Fig.  79. 


fore  capable  of  reflection.     The  calorific  rays  which  strike 
the  interior  surface  obliquely,  are  reflected  from  side  to  side 


BEFLECTION  FEOM  CURVED  SURFACES.       283 

of  the  tube,  and  thus  those  rays  which,  when  the  tube  is 
absent,  are  squandered  in  space,  are  caused,  by  internal  re- 
flection, to  reach  the  pile.  You  see  the  result :  the  needle, 
which  a  moment  ago  showed  no  sensible  action,  moves 
promptly  to  its  stops. 

We  have  now  dwelt  sufficiently  long  on  the  reflection  of 
radiant  heat  by  plane  surfaces  y  let  us  turn  for  a  moment  to 
reflection  from  curved  surfaces.  I  have  here  a  concave 
mirror,  M  N  (fig.  80)  formed  of  copper,  but  coated  with  sil- 

Fig.  80. 


ver.  I  place  this  warm  copper  ball,  B,  at  a  distance  of 
eighteen  inches  from  the  pile,  which  has  now  its  conical 
reflector  removed ;  you  observe  scarcely  any  motion  of  the 
needle.  If  I  placed  the  reflector,  M  N,  properly  behind  a 
candle,  I  should  collect  its  rays,  and  send  them  back  in  a  cyl- 
inder of  light.  I  shall  do  the  same  with  the  calorific  rays 
emitted  by  the  ball  B  ;  you  cannot,  of  course,  see  the  track 
of  these  obscure  rays,  as  you  can  that  of  the  luminous 
ones ;  but  you  observe  that  while  I  speak,  the  galvano- 
meter has  revealed  the  action ;  the  needle  of  the  instrument 
has  gone  up  to  90°. 


284:  LECTURE   VIII. 

I  have  here  a  pair  of  much  larger  mirrors,  one  of  which 
is  placed  flat  upon  the  table :  now,  the  curvature  of  this 
mirror  is  so  regulated  that  if  I  place  a  light  at  this  point, 
which  is  called  the  focus  of  the  mirror,  the  rays  which  fall 
divergent  upon  the  mirror  are  reflected  upward  from  it  par- 
allel.  Let  us  make  the  experiment :  In  the  focus  I  place 
our  coal-points,  bring  them  into  contact,  and  then  draw 
them  a  little  apart ;  there  is  the  electric  light,  and  there  is 
a  splendid  vertical  cylinder,  cast  upwards  by  the  reflector, 
and  marked  by  the  action  of  the  light  on  the  dust  of  the 
room.  If  we  reversed  the  experiment,  and  allowed  a  par- 
allel beam  of  light  to  fall  upon  the  mirror,  the  rays  of  that 
beam,  after  reflection,  would  be  collected  in  the  focus  of 
the  mirror.  We  can  actually  make  this  experiment  by  in- 
troducing a  second  mirror ;  here  it  is  suspended  from  the 
ceiling.,  I  will  now  draw  it  up  to  a  height  of  20  or  25  feet 
above  the  table  ;  the  vertical  beam,  which  before  fell  upon 
the  ceiling,  is  now  received  by  the  upper  mirror ;  I  have 
hung  in  the  focus  of  the  upper  mirror  a  bit  of  oiled  paper, 
to  enable  you  to  see  the  collection  of  the  rays  of  the  focus. 
You  observe  how  intensely  that  piece  of  paper  is  now  illu- 
minated, not  by  the  direct  light  from  below,  but  by  the  re- 
flected light  converged  upon  it  from  above. 

Many  of  you  know  the  extraordinary  action  of  light 
upon  a  mixture  of  hydrogen  and  chlorine.  I  have  here  a 
transparent  collodion  balloon  filled  with  the  mixed  gases  ; 
I  lower  my  upper  reflector,  and  suspend  the  balloon  from 
a  hook  attached  to  it,  so  that  the  little  globe  shall  swing 
in  the  focus  ;  we  will  now  draw  the  mirror  quite  up  to  the 
ceiling  (fig.  81)  ;  and  as  before  I  place  my  coal-points  in 
the  focus  of  the  lower  mirror  ;  the  moment  I  draw  them 
apart,  the  light  gushes  from  them,  and  the  gases  explode. 
And  remember  this  is  the  action  of  the  light ;  you  know 
collodion  to  be  an  inflammable  substance,  and  hence  might 
suppose  that  it  was  the  heat  of  the  coal-points  that  ignited 


CONJUGATE  MIKKOKS. 


285 


it,  and  that  it  commu- 
nicated its  combustion 
to  the  gases ;  but  look 
here !  you  see,  as  I 
speak,  the  flakes  of  the 
balloon  descending  on 
the  table;  the  lumi- 
nous rays  went  harm- 
lessly through  it,  caus- 
ed the  gases  to  ex- 
plode, and  the  hydro- 
chloric acid,  formed 
by  their  combustion, 
has  actually  preserved 
the  inflammable  envel- 
ope from  sharing  in 
the  combustion. 

I  lower  the  upper 
mirror  and  hang  in  its 
focus  a  second  balloon, 
containing  a  mixture 
of  oxygen  and  hydro- 
gen, on  which  light 
has  no  sensible  effect ; 
I  raise  the  mirror,  and 
in  the  focus  of  the 
lower  one  place  this 
red-hot  copper  ball. 
The  calorific  rays  are 
now  reflected  and  con- 
verged above,  as  the 
luminous  ones  were 
reflected  and  converg- 
ed in  the  last  experi- 
ment ;  but  they  act 


' 


286  LECTUBE  VIII. 

upon  the  envelope,  which  I  have  purposely  blackened  a  lit- 
tle, so  as  to  enable  it  to  intercept  the  heat-rays  ;  the  action 
is  not  so  sudden  as  in  the  last  case,  but  there  is  the  explo- 
sion, and  you  now  see  no  trace  of  the  balloon ;  the  inflam- 
mable substance  is  entirely  dissipated. 

But  here,  you  may  object,  light  is  associated  with  the 
heat ;  very  well,  I  lower  the  upper  mirror  once  more  and 
suspend  in  its  focus  a  flask  of  hot  water.  I  bring  my  ther- 
mo-electric pile  to  the  focus  of  the  lower  mirror,  and  first 
turn  the  face  of  the  pile  upwards,  so  as  to  expose  it  to  the 
direct  radiation  of  the  warm  flask — there  is  no  sensible  ac- 
tion produced  by  the  direct  rays.  But  I  now  turn  my  pile 
with  its  face  downwards.  If  light  and  heat  behave  alike, 
the  rays  from  the  flask  which  strike  the  reflector  will  be 
collected  at  its  focus.  You  see  that  this  is  the  case ;  the 
needle,  which  was  not  sensibly  affected  by  the  direct  rays, 
goes  up  to  its  stops.  I  would  ask  you  to  observe  the  di- 
rection of  that  deflection ;  the  red  end  of  the  needle  moves 
towards  you. 

I  again  lower  the  mirror,  and,  in  the  place  of  the  flask 
of  hot  water,  suspend  a  second  one  containing  a  freezing 
mixture.  I  raise  the  mirror  and,  as  in  the  former  case, 
bring  the  pile  into  the  focus  of  the  lower  one.  Turned  di- 
rectly towards  the  upper  flask  there  is  no  action ;  turned 
downwards,  the  needle  moves  :  observe  the  direction  of  the 
motion — the  red  end  com.es  towards  me. 

Does  it  not  appear  as  if  this  body  in  the  upper  focus 
were  now  emitting  rays  of  cold  which  are  converged  by 
the  lower  mirror  exactly  as  the  rays  of  heat  in  our  former 
experiment.  The  facts  are  exactly  complementary,  and  it 
would  seem  that  we  have  precisely  the  same  right  to  infer 
from  the  experiments,  the  existence  and  convergence  of 
these  cold  rays,  as  we  have  to  infer  the  existence  and  con- 
vergence of  the  heat  rays.  But  many  of  you,  no  doubt, 
have  already  perceived  the  real  state  of  the  case.  The  pile 


RADIATION  OF  COLD.  287 

is  a  warm  body,  but  in  the  last  experiment  the  quantity 
which  it  lost  by  radiation  was  more  than  made  good  by 
the  quantity  received  from  the  hot  flask  above.  Now  the 
case  is  reversed,  the  quantity  which  the  pile  radiates  is  in 
excess  of  the  quantity  which  it  receives,  and  hence  the  pile 
is  chilled ; — the  exchanges  are  against  it,  its  loss  of  heat 
is  only  partially  compensated — and  the  deflection  due  to 
cold  is  the  necessary  consequence. 


APPENDIX   TO   LECTURE  VIII. 


ON   THE   SOUNDS    PRODUCED  BY  THE    COMBUSTION  OF   GASES 
IN    TUBES.* 

IN  the  first  volume  of  Nicholson's  Journal,  published  in  1802,  the 
sounds  produced  by  the  combustion  of  hydrogen  in  tubes  are 
referred  to  as  having  been  '  made  in  Italy  : '  Dr.  Higgins,  in  the 
same  place,  shows  that  he  had  discovered  them  in  the  year  1777, 
while  observing  the  water  formed  in  a  glass  vessel  by  the  slow 
combustion  of  a  slender  stream  of  hydrogen.  Chladni,  in  his 
'Akustik,'  published  in  1802,  page  74,  speaks  of  their  being 
mentioned,  and  incorrectly  explained,  by  De  Luc  in  his  'New 
Ideas  on  Meteorology : '  I  do  not  know  the  date  of  the  volume. 
Chladni  himself  showed  that  the  tones  produced  were  the  same 
as  those  of  an  open  pipe  of  the  same  length  as  the  tube  which 
encompassed  the  flame.  He  also  succeeded  in  obtaining  a  tone 
and  its  octave  from  the  same  tube,  and  in  one  case  obtained  the 
fifth  of  the  octave.  In  a  paper  published  in  the  '  Journal  de  Phy- 
sique '  in  1802,  G.  De  la  Rive  endeavoured  to  account  for  the 
sounds  by  referring  them  to  the  alternate  contraction  and  expan- 
sion of  aqueous  vapour ;  basing  his  opinion  upon  a  series  of  ex- 
periments of  great  beauty  and  ingenuity  made  with  the  bulbs  of 
thermometers.  In  1818  Mr.  Faraday  took  up  the  subject,t  and 
showed  that  the  tones  were  produced  when  the  glass  tube  was 
enveloped  by  an  atmosphere  higher  in  temperature  than  212° 
Fahr.  That  they  were  not  due  to  aqueous  vapour  was  further 
shown  by  the  fact  that  they  could  be  produced  by  the  combustion 

*  From  the  Philosophical  Magazine  for  July,  1857.     By  John  Tyndall, 
F.R.S. 

f  Journal  of  Science  and  the  Arts,  vol.  v.  p.  274. 


SINGING  FLAMES.  289 

of  carbonic  oxide.  He  referred  the  sounds  to  successive  explo- 
sions produced  by  the  periodic  combination  of  the  atmospheric 
oxygen  with  the  issuing  jet  of  hydrogen  gas. 

I  am  not  aware  that  the  dependence  of  the  pitch  of  the  note 
on  the  size  of  the  flame  has  as  yet  been  noticed.  To  this  point  I 
will,  in  the  first  place,  briefly  direct  attention. 

A  tube  25  inches  long  was  placed  over  an  ignited  jet  of  hydro- 
gen :  the  sound  produced  was  the  fundamental  note  of  the  tube. 

A  tube  12£  inches  long  was  brought  over  the  same  flame,  but 
no  sound  was  obtained. 

The  flame  was  lowered,  so  as  to  make  it  as  small  as  possible, 
and  the  tube  last  mentioned  was  again  brought  over  it ;  it  gave 
a  clear  melodious  note,  which  was  the  octave  of  that  obtained 
with  the  25-inch  tube. 

The  25-inch  tube  was  now  brought  over  the  same  flame ;  it  no 
longer  gave  its  fundamental  note,  but  exactly  the  same  note  as 
that  obtained  from  the  tube  of  half  its  length. 

Thus  we  see,  that  although  the  speed  with  which  the  explo- 
sions succeed  each  other  depends  upon  the  length  of  the  tube, 
the  flame  has  also  a  voice  in  the  matter :  that  to  produce  a  musi- 
cal sound,  its  size  must  be  such  as  to  enable  it  to  explode  in 
unison  either  with  the  fundamental  pulses  of  the  tube,  or  with 
the  pulses  of  its  harmonic  divisions. 

With  a  tube  6  feet  9  inches  long,  by  varying  the  size  of  the 
flame,  and  adjusting  the  depth  to  which  it  reached  within  the 
tube,  I  have  obtained  a  series  of  notes  in  the  ratio  of  the  numbers 
1,  2,  3,  4,  5. 

These  experiments  explain  the  capricious  nature  of  the  sounds 
sometimes  obtained  by  lecturers  upon  this  subject.  It  is,  how- 
ever, always  possible  to  render  the  sounds  clear  and  sweet,  by 
suitably  adjusting  the  size  of  the  flame  to  the  length  of  the  tube.* 

Since  the  experiments  of  Mr.  Faraday,  nothing,  that  I  am 
aware  of,  has  been  added  to  this  subject,  until  quite  recently. 
In  a  recent  number  of  Poggendorff  s  *  Annalen '  an  interesting 

*  With  a  tube  14^  inches  in  length  and  an  exceedingly  minute  jet  of 
gas,  I  obtained,  without  altering  the  quantity  of  gas,  a  note  and  its  octave : 
the  flame  possessed  the  power  of  changing  its  own  dimensions  to  suit  both 
notes. 

13 


290  APPENDIX  TO   LECTUKE   VIII. 

experiment  is  described  by  M.  von  Schaftgotsch,  and  made  the 
subject  of  some  remarks  by  Prof.  Poggendorff  himself.  A  musical 
note  was  obtained  with  a  jet  of  ordinary  coal-gas,  and  it  was 
found  that  when  the  voice  was  pitched  to  the  same  note,  the 
flame  assumed  a  lively  motion,  which  could  be  augmented  until 
the  flame  was  actually  extinguished.  M.  von  Schaftgotsch  does 
not  describe  the  conditions  necessary  to  the  success  of  his  experi- 
ment ;  and  it  was  while  endeavouring  to  find  out  these  condi- 
tions that  I  alighted  upon  the  facts  which  form  the  principal 
subject  of  this  brief  notice.  I  may  remark  that  M.  von  Schaff- 
gotsch's  result  may  be  produced,  with  certainty,  if  the  gas  be 
caused  to  issue  under  sufficient  pressure  through  a  very  small 
orifice. 

In  the  first  experiments  I  made  use  of  a  tapering  brass  burner, 
10^  inches  long,  and  having  a  superior  orifice  about  gVkh.  of  an 
inch  in  diameter.  The  shaking  of  the  singing  flame  within  the 
glass  tube,  when  the  voice  was  properly  pitched,  was  so  manifest 
as  to  be  seen  by  several  hundred  people  at  once. 

I  placed  a  syrene  within  a  few  feet  of  the  singing-flame,  and 
gradually  heightened  the  note  produced  by  the  instrument.  As 
the  sounds  of  the  flame  and  syrene  approached  perfect  unison,  the 
flame  shook,  jumping  up  and  down  within  the  tube.  The  inter- 
val between  the  jumps  became  greater  until  the  unison  was  per- 
fect, when  the  motion  ceased  for  an  instant ;  the  syrene  still  in- 
creasing in  pitch,  the  motion  of  the  flame  again  appeared,  the 
jumping  became  quicker  and  quicker,  until  finally  it  escaped 
cognisance  by  the  eye. 

This  experiment  showed  that  the  jumping  of  the  flame,  ob- 
served by  M.  von  Schaffgotsch,  is  the  optical  expression  of  the 
leats  which  occur  at  each  side  of  the  perfect  unison :  the  beats 
could  be  heard  in  exact  accordance  with  the  shortening  and 
lengthening  of  the  flame.  Beyond  the  region  of  these  beats,  in 
both  directions,  the  sound  of  the  syrene  produced  no  visible 
motion  of  the  flame.  What  is  true  of  the  syrene  is  true  of  the 
voice. 

While  repeating  and  varying  these  experiments,  I  once  had  a 
silent  flame  within  a  tube,  and  on  pitching  my  voice  to  the  note 
of  the  tube,  the  flame,  to  my  great  surprise,  instantly  started  into 
song.  Placing  the  finger  on  the  end  of  the  tube,  and  silencing 


SINGING  FLAMES.  291 

the  melody,  on  repeating  the  experiment  the  same  result  was 
obtained. 

I  placed  the  syrene  near  the  flame,  as  before.  The  latter  was 
burning  tranquilly  within  its  tube.  Ascending  gradually  from 
the  lowest  notes  of  the  instrument,  at  the  moment  when  the  sound 
of  the  syrene  reached  the  pitch  of  the  tube  which  surrounded  the 
gas  flame,  the  latter  suddenly  stretched  itself  and  commenced  its 
song,  which  continued  indefinitely  after  the  syrene  had  ceased  to 
sound. 

With  the  burner  which  I  have  described,  and  a  glass  tube  12 
inches  long,  and  from  £  to  £  of  an  inch  internal  diameter,  this  re- 
sult can  be  obtained  with  ease  and  certainty.  If  the  voice  be 
thrown  a  little  higher  or  lower  than  the  note  due  to  the  tube,  no 
visible  effect  is  produced  upon  the  flame  :  the  pitch  of  the  voice 
must  lie  within  the  region  of  the  audible  beats. 

By  varying  the  length  of  the  tube  we  vary  the  note  produced, 
and  the  voice  must  be  modified  accordingly. 

That  the  shaking  of  the  flame,  to  which  I  have  already  re- 
ferred, proceeds  in  exact  accordance  with  the  beats,  is  beautifully 
shown  by  a  tuning-fork,  which  gives  the  same  note  as  the  flame. 
Loading  the  fork  so  as  to  throw  it  slightly  out  of  unison  with  the 
flame,  when  the  former  is  sounded  and  brought  near  the  flame, 
the  jumpings  are  seen  at  exactly  the  same  intervals  as  those  in 
which  the  beats  are  heard.  When  the  tuning-fork  is  brought 
over  a  resonant  jar  or  bottle,  the  beats  may  be  heard  and  the 
jumpings  seen  by  a  thousand  people  at  once.  By  changing  the 
load  upon  the  tuning-fork,  or  by  slightly  altering  the  size  of  the 
flame,  the  quickness  with  which  the  beats  succeed  each  other 
may  be  changed,  but  in  all  cases  the  j  limpings  address  the  eye  at 
the  same  moment  that  the  beats  address  the  ear. 

With  the  tuning-fork  I  have  obtained  the  same  results  as  with 
the  voice  and  syrene.  Holding  a  fork  over  a  tube  which  responds 
to  it,  and  which  contains  within  it  a  silent  flame  of  gas,  the  latter 
immediately  starts  into  song.  I  have  obtained  this  result  with  a 
series  of  tubes  varying  from  10^  to  29  inches  in  length.  The  fol- 
lowing experiment  could  be  made  : — A  series  of  tubes,  capable«of 
producing  the  notes  of  the  gamut,  might  be  placed  over  suitable 
jets  of  gas ;  all  being  silent,  let  the  gamut  be  run  over  by  a 
musician  with  an  instrument  sufficiently  powerful,  placed  at  a 


292  APPENDIX  TO   LECTURE  VIII. 

distance  of  twenty  or  thirty  yards.  At  the  sound  of  each  partic- 
ular note,  the  gas-jet  contained  in  the  corresponding  tube  would 
instantly  start  into  song. 

I  must  remark,  however,  that  with  the  jet  which  I  have  used, 
the  experiment  is  most  easily  made  with  a  tube  about  11  or  12 
inches  long  :  with  longer  tubes  it  is  more  difficult  to  prevent  the 
flame  from  singing  spontaneously,  that  is,  without  external  exci- 
tation. 

The  principal  point  to  be  attended  to  is  this.  With  a  tube, 
say  of  12  inches  in  length,  the  flame  requires  to  occupy  a  certain 
position  in  the  tube  in  order  that  it  shall  sing  with  a  maximum 
intensity.  Let  the  tube  be  raised  so  that  the  flame  may  penetrate 
it  to  a  less  extent ;  the  energy  of  the  sound  will  be  thereby 
diminished,  and  a  point  (A)  will  at  length  be  attained,  where  it 
will  cease  altogether.  Above  this  point,  for  a  certain  distance, 
the  flame  may  be  caused  to  burn  tranquilly  and  silently  for  any 
length  of  time,  but  when  excited  by  the  voice  it  will  sing. 

When  the  flame  is  too  near  the  point  (A),  on  being  excited  by 
the  voice  or  by  a  tuning-fork,  it  will  respond  for  a  short  time, 
and  then  cease.  A  little  above  the  point  where  this  cessation 
occurs,  the  flame  burns  tranquilly,  if  unexcited,  but  if  once  caused 
to  sing  it  will  (xmtniue  to  sing.  With  such  a  flame,  which  is  not 
too  sensitive  to  external  impressions,  I  have  been  able  to  reverse 
the  effect  hitherto  described,  and  to  stop  the  song  at  pleasure  by  the 
sound  of  my  voice,  or  by  a  tuning-fork,  without  quenching  the 
flame  itself.  Such  a  flame,  I  find,  may  be  made  to  obey  the  word 
of  command,  and  to  sing  or  cease  to  sing,  as  the  experimenter 
pleases. 

The  mere  clapping  of  the  hands,  producing  an  explosion, 
shouting  at  an  incorrect  pitch,  shaking  of  the  tube  surrounding 
the  flame,  are,  when  the  arrangements  are  properly  made,  ineffec- 
tual. Each  of  these  modes  of  disturbance  doubtless  affects  the 
flame,  but  the  impulses  do  not  accumulate,  as  in  the  case  where 
the  note  of  the  tube  itself  is  struck.  It  appears  as  if  the  flame 
were  deaf  to  a  single  impulse,  as  the  tympanum  would  probably 
be^  and,  like  the  latter,  needs  the  accumulation  of  impulses  to 
give  it  sufficient  motion.  A  difference  of  half  a  tone  between 
two  tuning-forks  is  sufficient  to  cause  one  of  these  to  set  the  flame 
singing,  while  the  other  is  powerless  to  produce  this  effect. 


SINGING  FLAMES.  293 

I  have  said  that  the  voice  must  be  pitched  to  the  note  of  the 
tube  which  surrounds  the  flame ;  it  would  be  more  correct  to  say 
the  note  produced  by  the  flame  when  singing.  In  all  cases  this 
note  is  sensibly  higher  than  that  due  to  the  open  tube  which  sur- 
rounds the  flame ;  this  ought  to  be  the  case,  because  of  the  high 
temperature  of  the  vibrating  column.  An  open  tube,  for  exam- 
ple, which,  when  a  tuning-fork  is  held  over  its  end,  gives  a  maxi- 
mum reinforcement,  produces,  when  surrounding  a  singing  flame, 
a  note  higher  than  that  of  the  fork.  To  obtain  the  latter  note 
the  tube  must  be  sensibly  longer. 

What  is  the  constitution  of  the  flame  of  gas  while  it  produces 
these  musical  sounds  ?  This  is  the  next  question  to  which  I  will 
briefly  call  attention.  Looked  at  with  the  naked  eye,  the  sound- 
ing flame  appears  constant,  but  is  the  constancy  real  ?  Supposing 
each  pulse  to  be  accompanied  by  a  physical  change  of  the  flame, 
such  a  change  would  not  be  perceptible  to  the  naked  eye,  on 
account  of  the  velocity  with  which  the  pulses  succeed  each  other. 
The  light  of  flame  would  appear  continuous,  on  the  same  princi- 
ple that  the  troubled  portion  of  a  descending  liquid  yet  appears 
continuous,  although  by  proper  means  this  portion  of  a  jet  can  be 
shown  to  be  composed  of  isolated  drops.  If  we  cause  the  image 
of  the  flame  to  pass  speedily  over  different  portions  of  the  retina, 
the  changes  accompany  the  periodic  impulses  will  manifest  them- 
selves in  the  character  of  the  image  thus  traced. 

I  took  a  glass  tube  3  feet  2  inches  long,  and  about  an  inch  and 
a  half  in  internal  diameter,  and  placing  it  over  a  very  small  flame 
of  olefiant  gas  (common  gas  will  also  answer),  obtained  the  fun- 
damental note  of  the  tube :  on  moving  the  head  to  and  fro,  the 
image  of  the  sounding  flame  was  separated  into  a  series  of  dis- 
tinct images;  the  distance  between  the  images  depended  upon 
the  velocity  with  which  the  head  was  moved.  This  experiment 
is  suited  to  a  darkened  kcture-room.  It  was  still  easier  to  obtain 
the  separation  of  the  images  in  this  way,  when  a  tube  6  feet  9 
inches  in  length,  and  a  large  flame,  were  made  use  of. 

The  same  result  is  obtained  when  an  opera  glass  is  moved  to 
and  fro  before  the  eye. 

But  the  most  convenient  mode  of  observing  the  flame  is  with 
a  mirror ;  and  it  can  be  seen  either  directly  in  the  mirror,  or  by 
projection  upon  a  screen. 


204  APPENDIX   TO   LECTURE   VIII. 

A  lens  of  33  centimetres  focus  was  placed  in  front  of  a  flame 
of  common  gas,  upwards  of  an  inch  long,  and  a  paper  screen  was 
hung  at  about  6  or  8  feet  distance  behind  the  flame.  In  front  of 
the  lens  a  small  looking-glass  was  held,  which  received  the  light 
that  had  passed  through  the  lens,  and  reflected  it  back  upon  the 
screen  placed  behind  the  latter.  By  adjusting  the  position  of  the 
lens,  a  well-defined  inverted  image  of  the  flame  was  obtained 
upon  the  screen.  On  moving  the  mirror*the  image  was  displaced, 
and  owing  to  the  retention  of  the  impression  by  the  retina,  when 
the'moveinent  was  sufficiently  speedy  the  image  described  a  con- 
tinuous luminous  track.  Holding  the  mirror  motionless,  the  C- 
foot  9-inch  tube  was  placed  over  the  flame :  the  latter  changed 
its  shape  the  moment  it  commenced  to  sound,  remaining  however 
well  defined  upon  the  screen.  On  now  moving  the  mirror,  a 
totally  different  effect  was  produced:  instead  of  a  continuous 
track  of  light,  a  series  of  distinct  images  of  the  sounding  flame 
was  observed.  The  distance  of  these  images  apart  varied  with 
the  motion  of  the  mirror ;  and,  of  course,  could  be  made,  by  suit- 
ably turning  the  reflector,  to  form  a  ring  of  images.  The  experi- 
ment is  beautiful,  and  in  a  dark  room  may  be  made  visible  to  a 
large  audience. 

The  experiment  was  also  varied  in  the  following  manner : — 
A  triangular  prism  of  wood  had  its  sides  coated  with  rectangular 
pieces  of  looking  glass :  it  was  suspended  by  a  thread  with  its 
axis  vertical ;  torsion  was  imparted  to  the  thread,  and  the  prism, 
acted  upon  by  this  torsion,  caused  to  rotate.  It  was  so  placed 
that  its  three  faces  received,  in  succession,  the  beam  of  light  sent 
from  the  flame  through  the  lens  in  front  of  it,  and  threw  the 
images  upon  the  screen.  On  commencing  its  motion  the  images 
were  but  slightly  separated,  but  became  more  and  more  so  as  the 
motion  approached  its  maximum.  This  once  past,  the  images 
drew  closer  together  again,  until  they  ended  in  a  kind  of  luminous 
ripple.  Allowing  the  acquired  torsion  to  react,  the  same  series  of 
effects  could  be  produced,  the  motion  being  in  an  opposite  direc- 
tion. In  these  experiments,  that  half  of  the  tube  which  was 
turned  towards  the  screen  was  coated  with  lamp-black,  so  as  to 
cut  off  the  direct  light  of  the  jet  from  the  screen.* 

*  Since  these  experiments  were  made,  Mr.  Wheatstone  has  drawn  my 


SINGING  FLAMES.  295 

But  what  is  the  state  of  the  flame  in  the  interval  between  two 
images  ?  The  flame  of  common  gas,  or  of  olefiant  gas,  owes  its 
luminousness  to  the  solid  particles  of  carbon  discharged  into  it. 
If  we  blow  against  a  luminous  gas-flame,  a  sound  is  heard,  a 
small  explosion  in  fact,  and  by  such  a  puff  the  light  may  be 
caused  to  disappear.  During  a  windy  night  the  exposed  gas-jets 
in  the  shops  are  often  deprived  of  their  light,  and  burn  blue.  In 
like  manner  the  common  blowpipe-jet  deprives  burning  coal-gas 
of  its  brilliant  light.  I  hence  concluded,  that  the  explosions,  the 
repetition  of  which  produces  the  musical  sound,  rendered,  at  the 
moment  they  occurred,  the  combustion  so  perfect  as  to  extin- 
guish the  solid  carbon  particles ;  but  I  imagined  that  the  images 
on  the  screen  would,  on  closer,  examination,  be  found  united  by 
spaces  of  blue,  which,  owing  to  their  dimness,  were  not  seen  by 
the  method  of  projection.  This  in  many  instances  was  found  to 
be  the  case. 

I  was  not,  however,  prepared  for  the  following  result : — A 
flame  of  olefiant  gas,  rendered  almost  as  small  as  it  could  be,  was 
procured.  The  3-foot  2-inch  tube  was  placed  over  it ;  the  flame, 
on  singing,  became  elongated,  and  lost  some  of  its  light,  still  it 
was  bright  at  its  top  ;  looked  at  in  the  moving  mirror,  a  beaded 
line  of  great  beauty  was  observed ;  in  front  of  each  bead  was  a 
little  luminous  star,  after  it,  and  continuous  with  it,  a  spot  of 
rich  blue  light,  which  terminated,  and  left,  as  far  as  I  could 
judge,  a  perfectly  dark  space  between  it  and  the  next  following 
luminous  star.  I  shall  examine  this  further  when  time  permits 
me,  but  as  far  as  I  can  at  present  judge,  the  flame  was  actually 
extinguished  and  relighted  in  accordance  with  the  sonorous  pul- 
sations. 

When  a  silent  flame,  capable,  however,  of  being  excited  by  the 
voice  in  the  manner  already  described,  is  placed  within  a  tube, 

attention  to  the  following  passage,  which  proves  that  he  had  already  made 
use  of  the  rotating  mirror  in  examining  a  singing  flame :  '  A  flame  of  hy- 
drogen gas  burning  in  the  open  air  presents  a  continuous  circle  in  the 
mirror ;  but  while  producing  a  sound  within  a  glass  tube,  regular  intermis- 
sions of  intensity  are  observed,  which  present  a  chain-like  appearance,  and 
indicate  alternate  contractions  and  dilatations  of  the  flame  corresponding 
with  the  sonorous  vibrations  of  the  column  of  air.' — Phil.  Trans.,  1834,  p. 
586. 


296  APPENDIX   TO   LECTURE   VIII. 

and  the  continuous  line  of  light  produced  by  it  in  the  moving 
mirror  is  observed,  I  know  no  experiment  more  pretty  than  the 
resolution  of  this  line  into  a  string  of  richly  luminous  pearls  at 
the  instant  the  voice  is  pitched  to  the  proper  note.  This  may  be 
done  at  a  considerable  distance  from  the  jet,  and  with  the  back 
turned  towards  it. 

The  change  produced  in  the  -line  of  beads  when  a  tuning-fork, 
capable  of  giving  beats  with  the  flame,  is  brought  over  the  tube, 
or  over  a  resonant  jar  near  it,  is  also  extremely  interesting  to 
observe.  I  will  not  at  present  enter  into  a  more  minute  descrip- 
tion of  these  results.  Sufficient,  I  trust,  has  been  said  to  induce 
experimenters  to  reproduce  the  effects  for  themselves ;  the  sight 
of  them  will  give  more  pleasure  than  any  description  of  mine 
could  possibly  do. 


TKANSLATION  OF  A  PAPEK  ON  ACOUSTIC  EXPERIMENTS.  * 

A  glass  tube  open  at  both  ends,  when  simply  blown  upon  by 
the  mouth,  gives  its  fundamental  tone,  i.  e.  the  deepest  tone  belong- 
ing to  it,  as  an  open  organ-pipe,  feebly  but  distinctly.  On  placing 
the  open  hand  upon  one  of  th  e  openings  and  rapidly  withdraw- 
ing it,  the  tube  yields  two  notes,  one  after  the  other ;  first  the 
fundamental  note  of  the  closed  pipe,  and  then  the  note  of  the 
open  pipe,  already  mentioned,  which  is  an  octave  higher.  By  the 
application  of  heat  these  fundamental  tones,  of  which  only  the 
higher  one  will  be  taken  into  consideration  here,  are  raised,  as  is 
well  known ;  this  is  observed  immediately  on  blowing  upon  a 
tube  heated  externally,  or  by  a  gas-flame  burning  in  its  interior. 
For  example,  a  tube  242  millims.  in  length,  and  20  millims.  in 
diameter,  heated  throughout  its  whole  length,  when  blown  upon 
even  before  it  reaches  a  red  heat,  gives  a  tone  raised  a  major 
third,  namely,  the  second  G  sharp  in  the  treble  clef,  instead  of  the 
corresponding  E.  If  a  gas-flame  14  millims.  in  length,  and  1 
millim.  in  breadth  at  the  bottom,  is  burning  in  the  tube,  the  tone 
rises  to  the  second  treble  F  sharp.  The  same  gas-flame  raises 

*  By  Count  Schaffgotsch :  Phil.  Mag.,  December  1857. 


SINGING  FLAMES.  297 

the  tone  of  a  tube  273  millims.  in  length,  and  21  millims.  in 
width,  from  the  second  treble  D  to  the  corresponding  E.  These 
two  tubes,  which  for  brevity  will  hereafter  be  referred  to  as  the 
E  tube  and  the  D  tube,  served  for  all  the  following  experiments, 
the  object  of  which  was  to  show  a  well-known  and  by  no  means 
surprising  fact,  in  a  striking  manner,  namely,  that  the  column 
of  air  in  a  tube  is  set  in  vibration  when  its  fundamental  tone,  or 
one  nearly  allied,  for  example,  an  octave,  is  sounded  outside  the 
tube.  The  existence  of  the  aerial  vibrations  was  rendered  per- 
ceptible by  a  column  of  smoke,  a  current  of  gas,  and  a  gas  flame. 

1.  A  glimmering  smoky  taper  was  placed  close  under  the  E 
tube  held  perpendicularly,  and  the  smoke  passed  through  the 
tube  in  the  form  of  a  uniform  thread.     At  a  distance  of  1*5  metre 
from  the  tube,  the  first  treble  E  was  sung.     The  smoke  curled, 
and  it  appeared  as  if  a  part  of  it  would  be  forced  out  at  the  upper, 
and  the  other  part  at  the  lower  opening  of  the  tube. 

2.  Two  gas-burners,  1  millim.  in  the  aperture,  were  applied 
near  each  other  to  the  same  conducting  tube.     Common  gas 
flowed  from  both  of  them ;  one  projected  from  below  into  the  D 
tube  for  about  one-fifth  of  its  length ;  the  gas  flame  of  the  other 
was  3  millims.  in  height.     At  a  distance  of  1'5  metre  therefrom 
the  first  treble  D  was  sung ;  the  flame  increased  several  times  in 
breadth  and  height,  and  consequently  in  size  generally ;  a  larger 
quantity  of  gas  therefore  flowed  out  of  the  outer  burner,  which  can 
only  be  explained  by  a  diminution  of  the  stream  of  gas  in  the 
inner  burner,  that  is,  in  the  one  surrounded  by  the  glass  tube. 

3.  A  burner,  with  an  aperture  of  1  millim.  projecting  from 
below  into  the  D  tube,  about  80  millims.,  yielded  a  gas  flame 
14  millims.  in  length.    At  5 '6  metres  therefrom  the  first  treble  E 
was  sung :  the  flame  was  instantaneously  extinguished.    The  same 
thing  took  place  at  7  metres,  when  the  flame  is  only  10  millims. 
in  height,  and  the  first  treble  D  sharp  is  sung. 

4.  The  last-mentioned  flame  is  also  extinguished  by  the  note  Q- 
sharp  sounded  close  to  it.    Noises,  such  as  the  clapping  of  hands, 
pushing  a  chair,  or  shutting  a  book,  do  not  produce  this  effect. 

5.  A  burner  with  an  aperture  of  0-5  millim.,  projecting  from 
below  60  millims.  into  the  D  tube,  yielded  a  globular  gas  flame 
3  to  3*5  millims.  in  diameter.    By  gradually  closing  a  stopcock 
the  passage  of  gas  was  more  and  more  limited.    The  flame  sud- 

13* 


298  APPENDIX  TO   LECTURE  VIII. 

dcnly  became  much  longer,  but  at  the  same  time  narrower,  and 
nearly  cylindrical,  acquiring  a  bluish  color  throughout,  and  from 
the  tube  a  piercing  second  treble  D  was  sounded ;  this  is  the 
phenomenon  of  the  so-called  chemical  harmonica,  which  has  been 
known  for  eighty  years.  "When  the  stopcock  is  still  further 
closed,  the  tone  becomes  stronger,  the  flame  longer,  narrower,  and 
nearly  spindle-shaped ;  at  last  it  disappears. 

An  effect  exactly  similar  to  that  caused  by  cutting  off  the 
gas  is  produced  upon  the  small  gas  flame  by  a  D,  or  the  first 
treble  D,  sung  or  sounded  from  instruments ;  and  in  this  case  it 
is  to  be  observed  that  the  flame  generally  becomes  the  more  sensi- 
tive the  smaller  it  is,  and  the  further  the  burner  projects  into  the 
glass  tube. 

6.  The  flame  in  the  D  tube  was  2  or  3  millims.  in  length ;  at 
a  distance  of  16'3  metres  (more  than  51  feet)  from  it,  the  first 
treble  D  was  sounded.     The  flame  immediately  acquired  the  un- 
usual form,  and  the  second  treble  D  sounded  and  continued  to 
sound  from  the  tube. 

7.  While  the  second  treble  D  of  the  preceding  experiment  was 
sounding,  the  first  treble  D  was  sounded  loudly  close  to  the  tube, 
when  the  flame  became  excessively  elongated,  and  then  disap- 
peared. 

8.  The  flame  being  only  1*5  millim.  in  length,  the  first  treble 
D  was  sounded.     The  flame  gave  out  the  second  treble  D  (and 
perhaps  sometimes  also  a  higher  D)  only  for  a  moment,  and  dis- 
appeared.    The  flame  is  also  affected  by  various  D's  of  an  ad- 
justible  labial  pipe,  by  the  contra  D,  D,  D,  the  first  treble  D,  and 
the  second  treble  D  of  a  harmonium,  but  by  no  single  C  sharp  or 
D  sharp  of  this  powerful  instrument.     It  is  also  affected  by  the 
third  treble  D  of  a  clarionet,  although  only  when  quite  close. 
The  sung  note  also  acts  when  it  is  produced  by  inspiration  (in  this 
case  the  second  treble),  or  when  the  mouth  is  turned  from  the 
flame. 

9.  In  immediate  proximity  the  note  G-  sung  is  effective. 
Some  influence  is  exerted  by  noises,  but  not  by  all,  and  often 

not  by  the  strongest  and  nearest,  evidently  because  the  exciting 
tone  is  not  contained  in  them. 

10.  The  flame  burning  quietly  in  the  interior  of  the  D  tube 
was  about  2'5  millims.  in  length.    In  the  next  room,  the  door  of 


SINGING  FLAMES.  299 

which  was  open,  the  four  legs  of  a  chair  were  stamped  simulta- 
neously upon  the  wooden  floor.  The  phenomenon  of  the  chemi- 
cal harmonica  immediately  occurred.  A  very  small  flame  is  of 
course  extinguished,  after  sounding  for  an  instant,  by  the  noise 
of  a  chair.  A  tambourine,  when  struck,  acts  sometimes,  but  in 
general  not. 

11.  The  flame  burning  in  the  excited  singing  condition  in  the 
interior  of  the  D  tube,  the  latter  was  slowly  raised  as  high  as 
possible  without  causing  the  return  of  the  flame  to  the  ordinary 
condition.    The  note,  the  first  treble  D,  was  sung  strongly  and 
1/roken  off  suddenly  at  a  distance  of   1-5  metre.    The  harmonic 
tone  ceased,  and  the  flame  fell  into  a  state  of  repose  without  being 
extinguished. 

12.  The  same  result  was  produced  by  acting  upon  the  draught 
of  air  in  the  tube  by  a  fanning  motion  of  the  open  hand  close 
above  the  upper  aperture  of  the  tube. 

13.  In  the  D  tube  there  were  two  burners  close  together ;  one 
of  them,  0-5  millirn.  in  aperture,  opened  5  millims.  below  the 
other,  the  diameter  of  which  was  1  millim.  or  more.     Currents 
of  gas,  independent  of  each  other,  flowed  out  of  both ;    that 
flowing  from  the  narrower  burner  being  very  feeble,  and  burning 
when  ignited,  with  a  flame  about  1*5  millim.  in  length,  nearly 
invisible  in  the  day ;  the  first  treble  D  was  sung  at  a  distance  of 
three  metres.    The  strong  current  of  gas  was  immediately  in- 
flamed,  because  the  little  flame  situated    below  it,  becoming 
elongated,  flared  up  into  it.     By  a  stronger  action  of  the  tone, 
the  small  flame  itself  is  extinguished,  so  that  an  actual  transfer 
of  the  flame  from  one  burner  to  the  other  takes  place.    Soon  after- 
wards the  feeble  current  of  gas  is  usually  again  inflamed  by  the 
large  flame,  and  if  the  latter  be  again  extinguished  alone,  every- 
thing is  ready  for  a  repetition  of  the  experiment. 

14.  The  same  result  is  furnished  by  stamping  with  the  chair, 
&c.    It  is  evident  that  in  this  way  gas-flames  of  any  desired 
size  and  any  mechanical  action  may  be  produced  by  musical 
tones  and  noises,  if  a  wire  stretched  by  a  weight  be  passed  through 
the  glass  tube  in  such  a  way  that  the  flaring  gas-flame  must  burn 
upon  it. 

15.  If  the  flame  of  the  chemical  harmonica  be  looked  at  stead- 
fastly, and  at  the  same  time  the  head  be  moved  rapidly  to  the 


300  APPENDIX  TO  LECTURE  VIH. 

right  and  left  alternately,  an  uninterrupted  streak  of  liglit  is  not 
seen,  such  as  is  given  by  every  other  luminous  body,  but  a  series 
of  closely  approximated  flames,  and  often  dentated  and  undulated 
figures,  especially  when  tubes  of  a  metre  and  flames  of  a  centi- 
metre in  length  are  employed. 

This  experiment  also  succeeds  very  easily  without  moving 
the  eyes,  when  the  flame  is  looked  at  through  an  opera-glass,  the 
object-glass  of  which  is  moved  rapidly  to  and  fro,  or  in  a  circle ; 
and  also  when  the  picture  of  the  flame  is  observed  in  a  hand- 
mirror  shaken  about.  It  is,  however,  only  a  variation  of  the 
experiment  long  since  described  and  explained  by  Wheatstonc, 
for  which  a  mirror  turned  by  watchwork  was  employed. 

[It  is  perhaps  but  right  that  I  should  draw  attention  to  the  relation  of 
the  foregoing  paper  to  one  that  I  have  published  on  the  same  subject.  On 
May  6,  and  the  days  immediately  following,  the  principal  facts  described  in 
my  paper  were  discovered ;  but  on  April  30,  the  foregoing  results  were 
communicated  by  Prof.  Poggendorff  to  the  Academy  of  Sciences  in  Berlin. 
Through  the  kindness  of  Mr.  Schaffgotsch  himself,  I  received  his  paper  at 
Chamouni,  many  weeks  after  the  publication  of  my  own,  and  until  then  I 
was  not  aware  of  his  having  continued  his  experiments  upon  the  subject. 

We  thus  worked  independently  of  each  other,  but  as  far  as  the  describ- 
ed phenomena  are  common  to  both,  all  the  merit  of  priority  rests  with 
Count  Schaffgotsch.— J.  T.] 


LECTURE    IX. 

[March  20,  1862.] 

LAW  OP  DIMINUTION  WITH  THE  DISTANCE — THE  WAVES  OP  SOUND  LONGI- 
TUDINAL ;  THOSE  OF  LIGHT  TRANSVERSAL — WHEN  THEY  OSCILLATE  THE 
MOLECULES  OP  DIFFERENT  BODIES  COMMUNICATE  DIFFERENT  AMOUNTS 
OF  MOTION  TO  THE  ETHER — RADIATION  THE  COMMUNICATION  OF  MO- 
TION TO  THE  ETHER  ;  ABSORPTION  THE  ACCEPTANCE  OF  MOTION  FROM 
THE  ETHER — THOSE  SURFACES  WHICH  RADIATE  WELL  ABSORB  WELL — A 
CLOSE  WOOLLEN  COVERING  FACILITATES  COOLING — PRESERVATIVE  IN- 
FLUENCE OP  GOLD-LEAF — THE  ATOMS  OF  BODIES  SELECT  CERTAIN  WAVES 
FOR  DESTRUCTION  AND  ALLOW  OTHERS  TO  PASS — TRANSPARENCY  AND 
DIATHERMANCY — DIATHERMIC  BODIES  BAD  RADIATORS — THE  TERM  QUAL- 
ITY AS  APPLIED  TO  RADIANT  HEAT — THE  RAYS  WHICH  PASS  WITHOUT 
ABSORPTION  DO  NOT  HEAT  THE  MEDIUM  :  THE  MOST  POWERFUL  SOLAR 
RAYS  MAY  PASS  THROUGH  AIR  WHILE  THE  AIR  REMAINS  BELOW  A 
FREEZING  TEMPERATURE— PROPORTION  OF  LUMINOUS  AND  OBSCURE  RAYS 
IN  VARIOUS  FLAMES. 

0 

I  HAVE  said  that  the  intensity  of  radiant  heat  dimin- 
ishes with  the  distance,  as  light  diminishes.  What  is 
the  law  of  diminution  for  light?  I  have  here  a  square 
sheet  of  paper,  each  side  of  the  square  measuring  two  feet ; 
I  fold  it  thus  to  form  a  smaller  square,  each  side  of  which 
is  a  foot  in  length.  The  electric  lamp  now  stands  at  a  dis- 
tance of  sixteen  feet  from  the  screen ;  at  a  distance  of 
eight  feet,  that  is  exactly  midway  between  the  screen  and 
the  lamp,  I  hold  this  square  of  paper ;  the  lamp  is  naked, 
unsurrounded  by  its  camera,  and  the  rays,  uninfluenced  by 
any  lens,  are  emitted  on  all  sides.  You  see  the  shadow  of 
the  square  of  paper  on  the  screen.  My  assistant  shall  meas- 
ure the  boundary  of  that  shadow,  and  now  I  unfold  my 
sheet  of  paper  so  as  to  obtain  the  original  large  square ; 


302 


LECTURE   IX. 


you  see  by  the  creases,  that  it  is  exactly  four  times  the  area 
of  the  smaller  one.  I  place  this  large  sheet  against  the 
screen,  and  find  that  it  exactly  covers  the  space  formerly 
occupied  by  the  shadow  of  the  small  square. 

On  the  small  square,  therefore,  when  it  stood  midway 
between  tbe  lamp  and  screen,  a  quantity  of  light  fell  which, 
when  the  small  square  is  removed,  is  diffused  over  four 
times  the  area  upon  the  screen.  But  if  the  same  quantity 
of  light  is  diffused  over  four  times  the  area,  it  must  be  dilu- 
ted to  one-fourth  of  its  original  intensity.  Hence,  by 
doubling  the  distance  from  the  source  of  light,  we  diminish 
the  intensity  to  one-fourth.  By  a  precisely  similar  mode 
of  experiment  we  could  prove,  that  by  trebling  the  dis- 
tance we  should  diminish  the  intensity  to  one-ninth ;  and 
by  quadrupling  the  distance  we  should  reduce  the  intensity 
to  one-sixteenth:  in  short,  we  thus  demonstrate  the  law 
that  the  intensity  of  light  diminishes  as  the  square  of  the 
distance  increases.  This  is  the  celebrated  law  of  Inverse 
Squares  as  applied  to  light. 

But  I  have  said  that  heat  diminishes  according  to  the 
same  law.  Observe  the  experiment  which  I  am  now  about 
to  perform  before  you.  I  have  here  a  tin  vessel ;  narrow, 
but  presenting  a  side  a  square  yard  in  area,  MN  (fig.  82). 
This  side,  you  observe,  I  have  coated  with  lampblack.  I 
fill  the  vessel  with  hot  water,  intending  to  make  this  large 
surface  my  source  of  radiant  heat.  I  now  place  the  conical 
reflector  on  the  thermo-electric  pile,  P,  but  instead  of  per- 
mitting it  to  remain  a  reflector,  I  push  into  the  hollow  cone 
this  lining  of  black  paper,  which  fits  exactly,  and  which, 
instead  of  reflecting  any  heat  that  may  fall  obliquely  on  it, 
completely  cuts  off  the  oblique  radiation.  The  pile  is  now 
connected  with  the  galvanometer,  and  I  place  its  reflector 
close  to  this  large  radiating  surface,  the  face  of  the  pile 
being  about  six  inches  distant  from  the  surface. 

The  needle  of  the  galvanometer  moves :  let  it  move 


DIMINUTION    WITH    DISTANCE. 


303 


until  it  takes  up  its  final  position.     It  now  points  steadily 
to  60°,  and  there  it  will  remain  as  long  as  the  temperature 


of  the  radiating  surface  remains  sensibly  constant.  I  will 
now  gradually  withdraw  the  pile  from  the  surface,  and  will 
ask  you  to  observe  the  effect  upon  the  galvanometer.  Of 
course  you  will  expect  that  as  I  retreat  from  the  source  of 
heat,  the  intensity  of  the  heat  will  diminish,  and  that  the 
deflection  of  the  galvanometer  will  diminish  in  a  corre- 
sponding degree.  I  am  now  at  double  the  distance,  but  the 
needle  does  not  move  ;  I  treble  the  distance,  the  needle  is 
still  stationary  ;  I  successively  quadruple,  quintuple — go  to 
ten  times  the  distance,  but  the  needle  is  rigid  in  its  adher- 
ence to  the  deflection  of  60°.  There  is,  to  all  appearance, 
no  diminution  at  all  of  intensity  with  the  increase  of  dis- 
tance. 

From  this  experiment,  which  might  at  first  sight  appear 
fatal  to  the  law  of  inverse  squares,  as  applied  to  heat,  Mel- 
loni,  in  the  most  ingenious  manner,  proved  the  law.  Mark 
his  reasoning.  I  again  place  the  pile  close  to  the  radiating 
surface.  Imagine  the  hollow  cone  in  front  of  the  pile  pro- 
longed ;  it  would  cut  the.  radiating  surface  in  a  circle,  and 


804  LECTUKE  IX. 

this  circle  is  the  only  portion  of  the  surface  whose  rays  can 
reach  the  pile.  All  the  other  rays  are  cut  off  by  the  non 
reflecting  lining  of  the  cone.  I  move  the  pile  to  double  the 
distance ;  the  section  of  the  cone  prolonged  now  encloses 
a  circle  of  the  radiating  surface,  exactly  four  times  the  area 
of  the  former  circle ;  at  treble  the  distance  the  radiating 
surface  is  augmented  nine  times  ;  at  ten  times  the  distance 
the  radiating  surface  is  augmented  100  times.  But  the 
constancy  of  the  deflection  proves  that  the  augmentation 
of  the  radiating  surface  must  be  exactly  neutralised  by  the 
diminution  of  intensity ;  the  radiating  surface  augments  as 
the  square  of  the  distance,  hence  the  intensity  of  the  heat 
must  dimmish  as  the  square  of  the  distance  ;  and  thus  the 
experiment,  which  might  at  first  sight  appear  fatal  to  the 
law,  demonstrates  the  law  in  the  most  simple  and  conclu- 
sive manner. 

Let  us  now  revert  for  a  moment  to  our  fundamental 
conceptions  regarding  radiant  heat.  Its  origin  is  an  oscil- 
latory motion  of  the  ultimate  particles  of  matter — a  motion 
taken  up  by  the  ether,  and  propagated  through  it  in  waves. 
The  particles  of  ether  in  these  waves  do  not  oscillate  in  the 
same  manner  as  the  particles  of  air  in  the  case  of  sound. 
The  air-particles  move  to  and  fro,  in  the  direction  in  which 
the  sound  travels,  the  ether  particles  move  to  and  fro, 
across  the  line  in  which  the  light  travels.  The  undulations 
of  the  air  are  longitudinal,  the  undulations  of  the  ether  are 
transversal.  The  ether  waves  resemble  more  the  ripples 
of  water  than  they  do  the  aerial  pulses  which  produce 
sound  ;  that  this  is  the  case  has  been  inferred  from  optical 
phenomena.  But  it  is  manifest  that  the  disturbance  pro- 
duced in  the  ether  must  depend  upon  the  character  of  the 
oscillating  mass  ;  one  atom  may  be  more  unwieldy  than  an- 
other, and  a  single  atom  could  not  be  expected  to  produce 
so  great  a  disturbance  as  a  group  of  atoms  oscillating  as  a 
system.  Thus,  when  different  bodies  are  heated,  we  may 


LAW  OF   INVERSE   SQUAR 


305 


fairly  expect  that  they  will  not  all  create  the  same  amount 
of  disturbance  in  the  ether.  It  is  probable  that  some  will 
communicate  a  greater  amount  of  motion  than  others :  in 
other  words,  that  some  will  radiate  more  copiously  than 
others ;  for  radiation,  strictly  defined,  is  the  communication 
of  motion  from  the  particles  of  a  heated  body,  to  the  ether 
in  which  these  particles  are  immersed. 

Let  us  now  test  this  idea  by  experiment.  I  have  here 
a  cubical  vessel,  c  (fig.  83) — a  '  Leslie's  cube ' — so  called 
from  its  having  been  used  by  Sir  John  Leslie  in  his  beauti- 
ful researches  on  radiant  heat.  The  mass  of  the  cube  is 
pewter,  but  one  of  its  sides  is  coated  with  a  layer  of  gold, 
another  with  a  layer  of  silver,  a  third  with  a  layer  of  cop- 
per, while  the  fourth  I  have  coated  with  a  varnish  of  isin- 
glass. I  fill  the  cube  with  hot  water,  and  keeping  it  at  a 
constant  distance  from  the  thermo-electric  pile,  P,  I  allow 

Fig.  83. 


its  four  faces  to  radiate,  in  succession,  against  the  pile. 
The  hot  gold  surface,  you  see,  produces  scarcely  any  deflec 
tion  ;  the  hot  silver  is  equally  inoperative,  the  same  is  the 
case  with  the  copper  ;  but  when  I  turn  this  varnished  sur- 


30G 


LECTCKE   IX. 


face  towards  the  pile,  the  gush  of  heat  becomes  suddenly 
augmented ;  and  the  needle,  as  you  see,  moves  up  to  its 
stops.  Hence  we  infer,  that  through  some  physical  cause 
or  other,  the  molecules  of  the  varnish,  when  set  in  motion 
by  the  hot  water  within  the  cube,  communicate  more  mo- 
tion to  the  ether  than  the  atoms  of  the  metals ;  in  other 
words,  the  varnish  is  a  better  radiator  than  the  metals  are. 
I  obtain  a  similar  result  when  I  compare  this  silver  teapot 
with  this  earthenware  one  ;  filling  them  both  with  boiling 
water,  the  silver,  you  see,  produces  but  little  effect,  while 
the  radiation  from  the  earthenware  is  so  copious  as  to  drive 
the  needle  up  to  90°.  Thus,  also,  if  I  compare  this  pewter 
pot  with  this  glass  beaker,  when  both  are  filled  with  hot 
water,  the  radiation  from  the  glass  is  much  more  powerful 
than  that  from  the  pewter. 

You  have  often  heard  of  the  effect  of  colours  on  radia- 
tion, and  heard  a  good  deal,  no  doubt,  which  is  unwarrant- 
ed by  experiment.  I  have  here  a  cube,  one  of  whose  sides 
is  coated  with  whiting,  another  with  carmine,  a  third  with 
lampblack,  while  the  fourth  is  left  uncoated.  I  present  the 
black  surface  first  to  the  pile,  the  cube  being  filled  with 
boiling  water  ;  the  needle  moves  up,  and  now  points  stead- 
ily to  65°.  The  cube  rests  upon  a  little  turn-table,  and  by 
turning  the  support  I  present  the  white  face  to  the  pile ; 
the  needle  remains  stationary,  proving  that  the  radiation 
from  the  white  surface  is  just  as  copious  as  that  from  the 
black.  I  turn  the  red  surface  towards  the  pile,  there  is  no 
change  in  the  position  of  the  needle.  I  turn  the  uncoated 
side,  the  needle  instantly  falls,  proving  the  inferiority  of 
the  metallic  surface  as  a  radiator.  I  repeat  precisely  the 
same  experiments  with  this  cube,  the  sides  of  which  are 
covered  with  velvet ;  one  face  with  black  velvet,  another 
with  white,  and  a  third  with  red.  The  results  are  precise- 
ly the  same  as  in  the  former  instances ;  the  three  velvet 
surfaces  radiate  alike,  while  the  naked  surface  radiates  less 


INFLUENCE   OF   COLOUKS   ON  RADIATION.  307 

than  any  of  them.  These  experiments  show  that  the  radia- 
tion from  the  clothes  which  cover  the  human  body,  is  inde- 
pendent of  the  colour  of  these  clothes ;  the  colour  of  an 
animal's  fur  is  equally  incompetent  to  influence  the  radia- 
tion. These  are  the  conclusions  arrived  at  by  Melloni  for 
obscure  heat.* 

But  if  the  coated  surface  communicates  more  motion  to 
the  ether  than  the  uncoated  one,  it  necessarily  follows  that 
the  coated  vessel  will  cool  more  quickly  than  the  uncoated 
one.  I  have  here  two  cubes,  one  of  which  is  quite  coated 
with  lampblack,  while  the  other  is  bright.  At  the  com- 
mencement of  the  lecture  I  poured  boiling  water  into  these 
vessels,  and  placed  in  each  a  thermometer.  A  short  time 
ago  both  thermometers  showed  the  same  temperature,  but 
now  one  of  them  is  two  degrees  below  the  other.  The  ve- 
locity of  cooling  in  one  vessel  is  greater  than  in  the  other, 
and  the  vessel  which  cools  quickest  is  the  coated  one.  Here 
are  two  vessels,  "one  of  which  is  bright  and  the  other  close- 
ly coated  with  flannel.  Half  an  hour  ago  two  thermometers 
plunged  in  these  vessels  showed  the  same  temperature,  but 
they  show  it  no  longer  ;  the  covered  vessel  has  now  a  tem- 
perature two  or  three  degrees  lower  than  the  naked  one. 
It  is  usual  to  preserve  the  heat  of  teapots  by  a  woollen 
covering,  but  the  cover  must  fit  very  loosely.  In  this  case, 
though  the  covering  may  be  a  good  radiator,  its  goodness 
is  more  than  counterbalanced  by  the  difficulty  encountered 
by  the  heat  in  reaching  the  outer  surface  of  the  covering. 
A  closely  fitting  cover  would,  as  we  have  seen,  promote  the 
loss  which  it  is  intended  to  diminish,  and  thus  do  more 
harm  than  good. 

One  of  the  most  interesting  points  connected  with  our 
subject  is  the  reciprocity  which  exists  between  the  power 


*  By  the  application  of  a  more  powerful  and  delicate  test  than  that 
employed  by  Melloni,  I  find  that  his  conclusions  will  require  modification. 


308 


LECTURE    IX. 


of  a  "body  to  communicate  motion  to  the  ether,  or  to  radi- 
ate ;  and  its  capacity  to  accept  motion  from  the  ether,  or 
to  absorb.  As  regards  radiation  we  have  already  compared 
lampblack  and  chalk  with  metallic  surfaces  ;  we  will  now 
compare  the  same  substances  with  reference  to  their  powers 
of  absorption.  I  have  here  two  sheets  of  tin,  M  N,  o  r  (fig. 
84),  one  of  them  coated  with  whiting  and  the  other  left 
uncoated.  I  place  them  thus  parallel  to  each  other,  and 

Fig.  84. 


at  a  distance  of  about  two  feet  asunder.  To  the  edge  of 
each  sheet  I  have  soldered  a  screw,  and  from  one  screw  to 
she  other  I  stretch  a  copper  wire,  a  #,  which  now  connects 
the  two  sheets.  At  the  back  of  the  sheet  I  have  soldered 
one  end  of  a  little  bar  of  bismuth,  to  the  other  end,  e,  of 
which  a  wire  is  soldered,  and  terminated  by  a  binding 
tcrew.  To  these  two  binding  screws  I  attach  the  two  ends 


RECIPROCITY   OF  RADIATION   AND  ABSORPTION.       309 

of  the  wire  coming  from  my  galvanometer  at  o,  and  you 
observe  I  have  now  an  unbroken  circuit,  in  which  the  gal- 
vanometer is  included.  You  know  already  what  the  bis- 
muth bars  are  intended  for.  I  place  my  warm  finger  on 
this  left-hand  one,  a  current  is  immediately  developed, 
which  passes  from  the  bismuth  to  the  tin,  thence  through 
the  wire  connecting  the  two  sheets,  thence  round  the  gal- 
vanometer, to  the  point  from  which  it  started.  You  ob- 
serve the  effect.  The  needle  of  the  galvanometer  moves 
through  a  large  arc  ;  the  red  end  going  towards  you.  The 
junction  of  tin  and  bismuth  is  now  cooling,  the  needle  re- 
turns to  0°,  and  now  I  will  place  my  finger  upon  the  bis- 
muth at  the  back  of  the  other  plate — you  see  the  effect — a 
large  deflection  in  the  opposite  direction ;  the  red  end  of 
the  needle  now  comes  towards  me.  I  withdraw  my  finger, 
the  junction  cools,  and  once  more  the  needle  sinks  to 
zero. 

I  set  this  stand  exactly  midway  between  the  two  sheets 
of  tin,  and  on  the  stand  I  intend  to  place  a  heated  copper 
ball ;  the  ball  will  radiate  its  heat  against  both  sheets ;  on 
the  right,  however,  the  rays  will  strike  upon  a  coated  sur- 
face, while  on  the  left  they  will  strike  upon  a  naked  metal- 
lic surface.  If  both  surfaces  drink  in  the  radiant  heat — if 
both  accept  with  equal  freedom  the  motion  of  the  ethereal 
waves — the  bismuth  junctions  at  the  backs  will  be  equally 
warmed,  and  one  of  them  will  neutralise  the  other.  But 
if  one  surface  be  a  more  powerful  absorber  than  the  other, 
that  which  absorbs  most  will  heat  its  bismuth  indicator 
most ;  a  deflection  of  the  galvanometer  needle  will  be  the 
consequence,  and  the  direction  of  the  deflection  will  tell  us 
which  is  the  best  absorber.  The  ball  is  now  upon  the 
stand,  and  you  see  we  have  not  long  to  wait  for  a  decision 
of  the  question.  The  prompt  and  energetic  deflection  of 
the  needle  informs  us  that  the  coated  surface  is  the  most 
powerful  absorber.  In  the  same  way  I  compare  lampblack 


310  LECTTJKE   IX. 

and  varnish  with  tin,  and  find  the  two  former  by  far  the 
best  absorbers.* 

The  thinnest  metallic  coating  furnishes  a  powerful  de- 
fence against  the  absorption  of  radiant  heat.  I  have  here 
a  sheet  of  '  gold  paper,'  the  gold  being  merely  copper  re- 
duced to  great  tenuity.  Here  is  a  red  powder,  the  iodide 
of  mercury,  with  which  I  coat  the  under  surface  of  the  gold 
paper.  This  iodide,  as  many  of  you.  know,  has  its  red  col- 
our discharged  by  heat,  the  powder  becoming  a  pale  yel- 
low. I  lay  the  paper  flat  on  this  board  with  the  coloured 
surface  downwards,  and  on  this  upper  metallic  surface  I 
paste  pieces  of  paper — common  letter  paper  will  answer  my 
purpose.  A  figure  of  any  desired  shape  is  thus  formed  on 
the  surface  of  the  copper.  I  now  take  a  red-hot  spatula  in 
my  hand  and  pass  it  several  times  over  the  sheet ;  the 
spatula  radiates  strongly  against  the  sheet,  but  I  apprehend 
this  its  rays  are  absorbed  in  very  different  degrees.  The 
metallic  surface  will  absorb  but  little ;  the  paper  surfaces 
will  absorb  greedily ;  and,  on-turning  up  the  sheet,  you  see 
the  effect :  the  iodide  underneath  the  metallic  portion  is 
perfectly  unchanged,  while  under  every  bit  of  paper  the 
colour  is  discharged,  thus  forming  below  an  exact  copy  of 
the  figure  pasted  on  the  opposite  surface  of  the  sheet. 
Here  is  another  example  of  the  same  kind,  for  which  I  am 
indebted  to  Mr.  Hill,  of  the  establishment  of  Mr.  Jacob 
Bell  in  Oxford  Street.  A  hot  fire  sent  its  rays  against  this 
painted  piece  of  wood  (fig.  85),  on  which  the  number  338 
was  printed  in  gold  leaf  letters  ;  the  paint  is  blistered  and 
charred  all  round  the  letters,  but  underneath  the  latter  the 
wood  and  paint  are  quite  unaffected.  This  thin  film  of 
gold  has  been  quite  sufficient  to  prevent  the  absorption,  to 
which  the  destruction  of  the  surrounding  surface  is  due. 

*  Colour,  according  to  Mclloni,  has  no  influence  on  the  absorption  of 
obscure  heat :  on  luminous  heat,  such  as  that  of  the  sun,  it  has  great  in- 
fluence. 


METALS   BAD  KADIATOKS   AND  ABSORBERS.        .    311 

The  luminiferous  ether  fills  stellar  space ;  it  makes  the 
universe  a  whole,  and  renders  the  intercommunication  of 
light  and  energy  between  star  and  star  possible.  But  the 
subtle  substance  penetrates  further  ;  it  surrounds  the  very 


atoms  of  solid  and  liquid  substances.  Transparent  bodies 
are  such,  because  the  ether  and  their  atoms  are  so  related 
to  each  other,  that  the  waves  which  excite  light  can  pass 
through  them,  without  transferring  their  motion  to  the 
atoms.  In  coloured  bodies  certain  waves  are  broken  or  ab- 
sorbed ;  but  those  which  give  the  body  its  colour  pass 
without  loss.  Through  this  solution  of  sulphate  of  copper, 
for  example,  the  blue  waves  speed  unimpeded,  but  the  red 
waves  are  destroyed.  I  form  a  spectrum  upon  the  screen  ; 
sent  through  this  solution  you  see  the  red  end  of  the  spec- 
trum is  cut  away.  This  piece  of  red  glass,  on  the  contrary, 
owes  its  redness  to  the  fact  that  its  substance  can  be  trav- 
ersed freely  by  the  longer  undulation  of  red,  while  the 
shorter  waves  are  absorbed.  Interposing  it  in  the  path  of 
this  light  you  see  it  cuts  the  blue  end  of  the  spectrum  quite 
away,  leaving  merely  a  vivid  red  band  upon  the  screen. 
This  blue  liquid  then  cuts  off  the  rays  which  are  transmit- 
ted by  the  red  glass  ;  and  the  red  glass  cuts  off  the  rays 
which  are  transmitted  by  the  liquid ;  by  the  union  of  both 
we  ought  to  have  perfect  opacity,  and  so  we  have.  When 


312  LECTURE   IX. 

both  are  placed  in  the  path  of  the  beam,  the  entire  spec- 
trum disappears  ;  the  union  of  these  two  transparent  bodies 
produce  an  opacity  equal  to  that  of  pitch  or  coal. 

I  have  here  another  liquid — a  solution  of  the  perman- 
ganate qf  potash — which  I  introduce  into  the  path  of  the 
beam.  See  the  effect  upon  the  spectrum ;  the  two  ends 
pass  freely  through,  you  have  the  red  and  the  blue,  but  be- 
tween both  a  space  of  intense  blackness.  The  yellow  of 
the  spectrum  is  pitilessly  destroyed  by  this  liquid  ;  through 
the  entanglement  of  its  atoms  these  yellow  rays  cannot 
pass,  while  the  red  and  the  blue  glide  round  them  and  get 
through  the  inter-atomic  spaces  without  sensible  hindrance. 
And  hence  the  gorgeous  colour  of  this  liquid.  I  will  turn 
the  lamp  round  and  project  a  disk  of  light  two  feet  in 
diameter  upon  the  screen.  I  now  introduce  this  liquid ; 
can  anything  be  more  splendid  than  the  colour  of  that  disk  ? 
I  again  turn  the  lamp  obliquely  and  introduce  a  prism ; 
here  you  have  the  components  of  that  beautiful  colour ;  the 
violet  component  has  slidden  away  from  the  red.  You  see 
two  definite  disks  of  these  two  colours  upon  the  screen, 
which  overlap  in  the  centre,  and  exhibit  there  the  colour  of 
the  composite  light  which  passes  through  the  liquid. 

Thus,  as  regards  the  waves  of  light,  bodies  exercise  as 
it  were  an  elective  power,  singling  out  certain  waves  for 
destruction,  and  permitting  others  to  pass.  Transparency 
to  one  wave  does  not  at  all  imply  transparency  to  others, 
and  from  this  we  might  reasonably  infer,  that  transparency 
to  light  does  not  imply  transparency  to  radiant  heat.  This 
conclusion  is  entirely  verified  by  experiment.  I  have  here 
a  tin  screen,  M  N  (fig.  86),  pierced  by  an  aperture,  behind 
which  is  soldered  a  small  stand  s.  I  place  this  copper  ball, 
B,  heated  to  dull  redness,  on  a  candlestick,  which  will  serve 
as  a  support  for  the  ball.  At  the  other  side  of  the  screen  I 
place  my  thermo-electric  pile,  P;  the  rays  from  the  ball 
now  pass  through  the  aperture  in  the  screen  and  fall  upon 


ELECTIVE  ABSORPTION. 


313 


tne  pile — the  needle  goes  up,  and  finally  comes  to  rest  with 
a  steady  deflection  of  80°.  I  have  here  a  glass  cell,  a 
quarter  of  an  inch  wide,  which  I  now  fill  with  distilled  wa- 
ter. I  place  the  cell  on  the  stand,  so  that  all  rays  reaching 
the  pile  must  pass  through  it;  what  takes  place?  The 


needle  steadily  sinks  almost  to  zero ;  scarcely  a  ray  from 
the  ball  can  cross  this  water ; — to  the  undulations  issuing 
from  the  ball  the  water  is  practically  opaque,  though  so 
extremely  transparent  to  the  rays  of  light.  Before  remov- 
ing the  cell  of  water  I  place  behind  it  a  similar  cell,  con- 
taining transparent  bisulphide  of  carbon ;  so  that  now, 
when  I  remove  the  water  cell,  the  aperture  is  still  barred 
by  the  new  liquid.  What  occurs  ?  The  needle  promptly 
moves  upwards  and  describes  a  large  arc ;  so  that  the  self- 
same rays  that  found  the  water  impenetrable,  find  easy  ac- 
cess through  the  bisulphide  of  carbon.  In  the  same  way  I 
compare  this  alcohol  with  this  chloride  of  phosphorus,  and 
find  the  former  almost  opaque  to  the  rays  emitted  by  our 
warm  ball,  while  the  latter  permits  them  to  pass  freely. 
So  also  as  regards  solid  bodies  ;  I  have  here  a  plate  of 
14 


314:  LECTURE    IX. 

very  pure  glass,  which  I  place  on  the  stand,  and,  using  a 
cube  of  hot  water  instead  of  the  ball  B,  I  permit  the  rays 
from  the  heated  cube  to  pass  through  it,  if  they  can.  No 
movement  of  the  needle  is  perceptible.  I  now  displace  the 
plate  of  glass  by  a  plate  of  rocksalt  of  ten  tunes  the  thick- 
ness ;  you  see  how  promptly  the  needle  moves,  until  it  is 
arrested  by  its  stops.  To  these  rays,  then,  the  rocksalt  is 
eminently  transparent,  while  the  glass  is  practically  opaque 
to  them. 

For  these,  and  numberless  similar  results,  we  are  in- 
debted to  Melloni,  who  may  be  almost  regarded  as  the 
creator  of  this  branch  of  our  subject.  To  express  this 
power  of  instantaneous  transmission  of  radiant  heat,  he 
proposes  the  word  diathermancy.  Diathermancy  bears  the 
same  relation  to  radiant  heat  that  transparency  does  to 
light.  Instead  of  giving  you  determinations  of  my  own 
of  the  diathermancy  of  various  bodies,  I  will  make  a  selec- 
tion from  the  tables  of  the  eminent  Italian  philosopher  just 
referred  to.  In  these  determinations  Melloni  uses  four  dif- 
ferent sources  of  heat,  the  flame  of  a  Locatelli  lamp ;  a 
spiral  of  platinum  wire,  kept  incandescent  by  the  flame  of 
an  alcohol  lamp ;  a  plate  of  copper  heated  to  400°  Cent., 
and  a  plate  of  copper  heated  to  100°  Cent.,  the  last  men- 
tioned source  being  the  surface  of  a  copper  cube  contain- 
ing boiling  water.  The  experiments  were  made  in  the  fol- 
lowing manner : — First,  the  radiation  of  the  source,  that  is 
to  say  the  galvanometeric  deflection  produced  by  it,  was 
determined  when  nothing  but  air  intervened  between  the 
source  and  the  pile  ;  then  the  substance  whose  diatherman- 
cy was  to  be  examined  was  introduced,  and  the  consequent 
deflection  noted.  Calling  the  quantity  of  heat  represented 
by  the  former  deflection  100,  the  proportionate  quantities 
transmitted  by  twenty-five  different  substances  are  given  in 
the  following  table : — 


DIATHERMANCY. 


315 


Transmissions  :  per  centago  of  the  total 

radiation. 

Names  of  substances  reduced  to    n 
common  thickness  of  ^tu  of  an  inch 

(2  6  inillim.) 

Locatelli 

Lamp 

Incan- 
descent 
Platinum 

Copper  at 
401)°  C. 

Copper  at 
100°  C. 

1  Rocksalt 

92-3 

92-3 

92-3 

92-3 

2  Sicilian  sulphur 

74 

77 

60 

54 

3  Fluor  spar     . 

72 

69 

42 

33 

4  Beryl 

54 

23 

13 

0 

5  Iceland  spar  . 

39 

28 

6 

0 

6  Glass 

39 

24 

6 

0 

7  Rock  crystal  (clear) 

38 

28 

6 

3 

8  Smoky  quartz 

37 

28  * 

6 

3 

9  Chromate  of  Potash 

34 

28 

15 

0 

10  White  Topaz 

33 

24 

4 

0 

1  1  Carbonate  of  Lead 

32 

23 

4 

0 

12  Sulphate  of  Baryta 

24 

18 

3 

0 

13  Felspar 

23 

19 

6 

0 

14  Amethyst  (violet) 

21 

9 

2 

0 

15  Artificial  amber 

21 

5 

0 

0 

IGBorate  of  Soda 

18 

12 

8 

0 

17  Tourmaline  (deep  gr  en) 

18 

16 

3 

0 

18  Common  gum 

18 

3 

0 

0 

19  Selenite 

14 

5 

0 

0 

20  Citric  acid     . 

11 

2 

0 

0 

21  Tartrate  of  Potash 

11 

3 

0 

0 

22  Natural  amber 

11 

5 

0 

0 

23  Alum 

9 

2 

0 

0 

24  Sugar-candy  . 

8 

1 

0 

0 

25  Ice    . 

6 

0-5 

0 

0 

This  table  shows,  in  the  first  place,  what  very  different 
transmissive  powers  different  solid  bodies  possess.  It 
shows  us  also  that,  with  a  single  exception,  the  transparen- 
cy of  the  bodies  mentioned  for  radiant  heat  varies  with  the 
quality  of  the  heat.  Rocksalt  alone  is  equally  transparent 
to  heat  from  the  four  sources  experimented  with.  It  must 
be  borne  in  mind  here  that  the  luminous  rays  are  also  calo- 
rific rays ;  that  the  selfsame  ray,  falling  upon  the  nerve 
of  vision,  produces  the  impression  of  light ;  while,  imping- 
ing upon  other  nerves  of  the  body,  it  produces  the  impres- 


316  LECTURE  IX. 

sion  of  heat.  The  luminous  calorific  rays  have,  however,  a 
shorter  length  than  the  obscure  rays,  and  knowing,  as  we 
do,  how  differently  waves  of  different  lengths  are  absorbed 
by  bodies,  we  are  in  a  measure  prepared  for  the  results  of 
the  foregoing  table.  Thus,  while  glass,  of  the  thickness 
specified,  permits  39  per  cent,  of  the  rays  of  Locatelli's 
lamp,  and  24  per  cent,  of  the  rays  from  the  incandescent 
platinum  to  pass,  it  gives  passage  to  only  6  per  cent,  of  the 
rays  from  copper,  at  a  temperature  of  400°  C.,  while  it  is 
absolutely  opaque  to  all  rays  emitted  from  a  source  of  100° 
C.  We  also  see  that  limpid  ice,  which  is  so  highly  trans- 
parent to  light,  allows  to  pass  only  6  per  cent,  of  the  rays 
of  the  lamp,  and  0*5  per  cent,  of  the  rays  emitted  by  the 
incandescent  platinum,  while  it  utterly  cuts  off  all  rays  issu- 
ing from  the  other  two  sources.  We  have  here  an  intima- 
tion, that  by  far  the  greater  portion  of  the  rays  emitted  by 
the  lamp  of  Locatelli  must  be  obscure.  Luminous  rays 
pass  through  ice,  of  the  thickness  here  given,  without  sen- 
sible absorption,  and  the  fact  that  94  per  cent,  of  the  rays 
issuing  from  Locatelli's  flame  are  destroyed  by  the  ice, 
proves  that  this  proportion  of  these  rays  must  be  obscure. 
As  regards  the  influence  of  transparency,  clear  and  smoky 
quartz  are  very  instructive.  Here  are  the  two  substances, 
one  perfectly  pellucid,  the  other  a  dark  brown ;  still,  for 
the  luminous  rays  only,  do  these  two  specimens  show  a 
difference  of  transmission.  The  clear  quartz  transmits  38 
per  cent.,  and  the  smoky  quartz  37  per  cent,  of  the  rays 
from  the  lamp,  while,  for  the  other  three  sources,  the  trans- 
missions of  both  substances  are  identical. 

Melloni  supposed  rocksalt  to  be  perfectly  transparent 
to  all  kinds  of  calorific  rays,  the  7*7  per  cent,  less  than  a 
hundred  which  the  foregoing  table  exhibits,  being  due, 
not  to  absorption  but  to  reflection  at  the  two  surfaces  of 
the  plate  of  salt.  But  the  accurate  experiments  of  MM. 
de  la  Provostaye  and  Desains  prove  that  this  substance  is 
permeable  in  different  degrees  to  heat  of  different  kinds ; 


RADIATION  THBOUGH   SOLIDS   AND   LIQUIDS. 


317 


Transmission  : 


Name  of  Liquids:  thickness.  0'36 

Bisulphide  of  carbon    .           .           .           .  .  •  •    & 

Bichloride  of  sulphur  .           .           .  •  •  .63 

Protochloride  of  phosphorus              .           .  .  •  •    62 

Essence  of  turpentine  .           .           .           .  .  •  •    81 

Olive  oil             ......  ,  .    30 

Naphtha  .........    28 

Essence  of  lavender     .           .           .           .  .  •  .26 

Sulphuric  ether             .           .           .           .  .  •  .21 

Sulphuric  acid  .           .           .           .           .  ->  .-•     .  •    If 

Hydrate  of  ammonia    .           .           .           .  •  ,  '  •'•  .  '  .15 

Nitric  acid         .....  .      .  .  .15 

Absolute  alcohol   .  15 


Hydrate  of  potash 
Acetic  acid         . 
Pyroligneous  acid 
Concentrated  solution 
Solution  of  rocksalt 
White  of  egg     . 
Distilled  water 


f  sugar 


while  Mr.  Balfour  Stewart  has  established  the  important 
fact,  that  rocksalt  is  particularly  opaque  to  rays  issuing 
from  a  heated  piece  of  the  same  substance. 

In  the  preceding  table,  which  I  also  borrow  from  Mel- 
loni,  the  caloric  transmissions  of  nineteen  different  liquids 
are  given.  The  source  of  heat  was  an  Argand  lamp,  fur- 
nished with  a  glass  chimney,  and  the  liquids  were  enclosed 
in  a  cell  with  glass  sides,  the  thickness  of  the  liquid  layer 
being  9*21  millimetres,  or  0*36  of  an  inch.  Liquids  are 
here  shown  to  be  as  diverse  in  their  powers  of  transmis- 
sion as  solids  ;  and  it  is  also  worthy  of  remark,  that  water 
maintains  its  opacity,  notwithstanding  the  change  in  its 
state  of  aggregation. 

The  reciprocity  which  we  have  already  demonstrated 
between  radiation  and  absorption  in  the  case  of  metals, 
varnishes,  &c.,  may  now  be  extended  to  the  bodies  contain- 
ed in  Melloni's  tables.  I  will  content  myself  with  one  or 
two  illustrations,  borrowed  from  Mr.  Balfour  Stewart.  Here 
is  a  copper  vessel  in  which  water-  is  kept  in  a  state  of  gen- 
tle ebullition.  On  the  flat  copper  lid  of  this  vessel  I  place 
plates  of  glass  and  of  rocksalt,  till  they  have  assumed  the 
temperature  of  the  lid.  I  place  the  plate  of  rocksalt  upon 


318  LECTUKE   IX. 

this  stand,  in  front  of  the  thermo-electric  pile.  You  ob- 
serve the  deflection ;  it  is  so  small  as  to  be  scarcely  sensi- 
ble. I  now  remove  the  rocksalt,  and  put  in  its  place  a 
plate  of  heated  glass  ;  the  needle  moves  upwards  through 
a  large  arc,  thus  conclusively  showing  that  the  glass,  which 
is  the  mors  powerful  absorber  of  obscure  heat,  is  also  the 
more  powerful  radiator.  Alum,  unfortunately,  melts  at  a 
temperature  lower  than  that  here  made  use  of;  but  though 
its  temperature  is  not  so  high  as  that  of  the  glass,  you  can 
see  that  it  transcends  the  glass  as  a  radiator ;  the  action  on 
the  galvanometer  is  still  more  energetic  than  in  the  case  of 
the  last  experiment. 

Absorption  takes  place  within  the  absorbing  body  ;  and 
it  requires  a  certain  thickness  of  the  body  to  accomplish 
the  absorption.  This  is  true  of  both  light  and  radiant 
heat.  A  very  thin  stratum  of  pale  beer  is  almost  as  colour- 
less as  a  stratum  of  water,  the  absorption  being  too  incon- 
siderable to  produce  the  decided  colour  which  larger  masses 
of  the  beer  exhibit.  I  pour  distilled  water  into  a  drinking 
glass  ;  in  this  quantity  it  exhibits  no  trace  of  colour,  but  I 
have  arranged  here  an  experiment  which  will  show  you  that 
this  pellucid  liquid,  in  sufficient  thickness,  exhibits  a  very 
decided  colour.  Here  is  a  tube  fifteen  feet  long,  A  B  (fig. 
87),  placed  horizontal,  the  ends  of  which  are  stopped  by 

Fig.  87. 


pieces  of  plate  glass ;  at  one  end  of  the  tube  stands  an  elec- 
tric lamp,  L,  from  which  I  intend  to  send  a  cylinder  of 
light  through  the  tube.  The  tube  is  now  half  filled  with 
water,  the  upper  surface  of  which  cuts  the  tube  in  two 


INFLUENCE   OF  THICKNESS. 


319 


equal  parts  horizontally.  Thus  I  send  half  of  my  beam 
through  air  and  half  through  water,  and  with  this  lens,  c, 
I  intend  to  project  a  magnified  image  of  the  adjacent  end 
of  the  tube,  upon  the  screen.  Here  it  is ;  you  see  the 
image,  o  p,  composed  of  two  semicircles,  one  of  which  is 
due  to  the  light  which  has  passed  through  the  water,  the 
other  to  the  light  which  has  passed  through  the  air.  Side 
by  side,  thus,  you  can  compare  them,  and  you  notice  that 
while  the  air  semicircle  is  a  pure  white,  the  water  semicir- 
cle is  a  bright  and  delicate  blue  green.  Thus,  by  augment- 
ing the  thickness  through  which  the  light  has  to  pass,  you 
deepen  the  colour ;  this  proves  that  the  destruction  of  the 
light  rays  takes  place  within  the  absorbing  body,  and  is  not 
an  effect  of  its  surface  merely. 

Melloni  shows  the  same  to  be  true  of  radiant  heat.  In 
our  table,  at  page  311,  the  thickness  of  the  plates  used  was 
2 *6  millimetres,  but  by  rendering  the  plate  thinner  we  en- 
able a  greater  quantity  of  heat  to  get  through,  and  by  ren- 
dering it  sufficiently  thin,  we  may,  with  a  very  opaque  sub- 
stance, almost  reach  the  transmission  of  rocksalt.  The  fol- 
lowing table  shows  the  influence  of  thickness  on  the  trans- 
missive  power  of  a  plate  of  glass. 


Thickness  of 
Tlates  in  Milli- 
metres 

Transmission  by  Glass  of  different  thicknesses;  per  centago  of 
the  total  Kadiation 

Locatelli  Lamp 

Incandescent 
Platinum 

Copper  at 
400°  C. 

Copper  at 
100°  C. 

2-6 

0-5 
0-07 

39 
54 

77 

24 

37 
57 

6 
12 
34 

0 
1 
12 

Thus,  we  see,  that  by  diminishing  the  thickness  of  the 
plate  from  2'6  to  T07  mllimetres,  the  quantity  of  heat 
transmitted  rises,  in  the  case  of  the  lamp  of  Locatelli,  from 


320 


LECTURE  IX. 


39  to  77  per  cent. ;  in  the  case  of  the  incandescent  plati- 
num, from  24  to  57  per  cent. ;  in  the  case  of  copper  at  400° 
C.  from  6  to  34  per  cent. ;  and  in  the  case  of  copper  at 
100°  C.,  from  absolute  opacity  to  a  transmission  of  12  per 
cent. 

The  influence  of  the  thickness  of  a  plate  of  selenite  on 
the  quantity  of  heat  which  it  transmits  is  exhibited  in  the 
following  table. 


Thickness  of 
Plates  in  Milli- 
metres. 

Transmissions  by  Selenite  of  different  thicknesses;  percentage 
of  total  radiation. 

Locatelli  Lamp 

Incandescent 
Platinum 

Copper  at 
400°  C. 

Copper  at 
106°  C. 

2.6 
0.4 
0.01 

14 

38 
64 

5 
18 
51 

0 
7 
32 

0 
0 
31 

The  decomposition  of  the  solar  beam  gives  us  the  solar 
spectrum ;  luminous  in  the  centre,  calorific  at  one  end,  and 
chemical  at  the  other.  The  sun  is  therefore  a  source  of 
heterogenous  rays,  and  there  can  scarcely  be  a  doubt  that 
all  other  sources  of  heat,  luminous  and  obscure,  partake  of 
this  heterogeniety.  In  general,  when  such  mixed  rays  enter 
a  diathermic  substance,  some  are  struck  down  and  others 
permitted  to  pass.  Supposing,  then,  that  we  take  a  sheaf 
of  calorific  rays  which  have  already  passed  through  a  dia- 
thermic plate,  and  permit  them  to  fall  upon  a  second  plate 
of  the  same  material,  the  transparency  of  this  second  plate 
to  the  heat  incident  upon  it  will  be  greater  than  the  trans- 
parency of  the  first  plate  to  the  heat  incident  on  it.  In 
fact  the  first  plate,  if  sufficiently  thick,  has  already  extin- 
guished, in  great  part,  the  rays  which  the  substance  is 
capable  of  absorbing ;  and  the  residual  rays,  as  a  matter  of 
course  penetrate  a  second  plate  of  the  same  substance  with 


SIFTING   OF   CALORIFIC   BEAMS.  321 

comparative  freedom.  The  original  beam  is  sifted  by  the 
first  plate,  and  the  purified  beam  possesses,  for  the  same 
substance,  a  higher  penetrative  power  than  the  original 
beam. 

This  power  of  penetration  has  usually  been  taken  as  a 
test  of  the  quality  of  heat ;  the  heat  of  the  purified  beam  is 
said  to  be  different  in  quality  from  that  of  the  unpurified 
beam.  It  is  not,  however,  that  any  individual  ray  has 
changed  its  quality,  but  that  from  the  beam,  as  a  whole, 
certain  rays  have  been  withdrawn,  and  that  their  with- 
drawal has  altered  the  proportion  of  the  incident  heat 
transmitted  by  a  second  substance.  This,  I  think,  is  the 
true  meaning  of  the  term  '  quality '  as  appled  to  radiant 
heat.  In  the  path  of  the  rays  from  a  lamp  let  plates  of 
rocksalt,  alum,  bichromate  of  potash,  and  selenite  be  suc- 
cessively placed,  each  plate  2'6  millimetres  in  thickness  ;  let 
the  heat  emergent  from  these  plates  fall  upon  a  second  se- 
ries of  the  same  thickness ;  out  of  every  100  rays  of  this 
latter  heat,  the  following  proportions  are  transmitted. 

Rocksalt          .  .  92-S 

Alum  .  .  90 

Chromatc  of  Potash  .  71 

Selenite  .  .  91 

Referring  to  the  table,  p.  311,  we  find  that  of  the 
whole  of  the  rays  emitted  by  the  Locatelli  lamp,  only  34 
per  cent,  are  transmitted  by  the  chromate  of  potash  ;  here 
we  find  the  percentage  71.  Of  the  entire  radiation,  sele- 
nite transmits  only  14  per  cent.,  but  of  the  beam  which  has 
been  purified  by  a  plate  of  its  own  substance  it  transmits 
91  per  cent.  The  same  remark  applies  to  the  alum,  which 
transmits  only  9  per  cent,  of  the  unpurified  beam,  and  90 
per  cent,  of  the  purified  beam.  In  rocksalt,  on  the  con- 
trary, the  transmissions  of  the  sifted  and  unsifted  beam 
are  the  same,  because  the  substance  is  equally  transparent 
14* 


322  LECTUEE   IX. 

to  rays  of  all  kinds.*  In  these  cases  I  have  supposed  the 
rays  emergent  from  rocksalt  to  pass  through  rocksalt ;  the 
rays  emergent  from  alum  to  pass  through  alum,  and  so  of 
the  others ;  but,  as  might  be  expected,  the  sifting  of  the 
beam,  by  any  substance,  will  alter  the  proportion  in  which 
it  will  be  transmitted  by  almost  any  other  second  sub- 
stance. 

I  will  conclude  these  observations  with  an  experiment 
which  will  show  you  the  influence  of  sifting  in  a  very 
striking  manner.  I  have  here  a  sensitive  differential  air- 
thermometer  with  a  clean  glass  bulb.  You  see  the  slight- 
est touch  of  my  hand  causes  a  depression  of  the  thermo- 
metric  column.  Here  is  our  electric  lamp,  and  from  it  I 
will  converge  a  powerful  beam  on  the  bulb  of  that  ther- 
mometer. The  focus  now  falls  directly  on  the  bulb,  and 
the  air  within  it  is  traversed  by  a  beam  of  intense  power ; 
but  not  the  slightest  depression  of  the  thermometric  col- 
umn is  discernible.  When  I  first  showed  this  experiment 
to  an  individual  here  present,  he  almost  doubted  the  evi- 
dence of  his  senses ;  but  the  explanation  is  simple.  The 
beam,  before  it  reaches  the  bulb,  is  already  sifted  by  the 
glass  lens  used  to  concentrate  it,  and  having  passed 
through  12  or  14  feet  of  air,  the  beam  contains  no  constit- 
uent that  can  be  sensibly  absorbed  by  the  air  within  the 
bulb.  Hence  the  hot  beam  passes  through  both  air  and 
glass  without  warming  either.  It  is  competent,  however, 
to  warm  the  thermo-electric  pile ;  exposure  of  the  pile  to 
it,  for  a  single  instant,  suffices  to  drive  the  needle  violently 
aside ;  or  let  me  coat  with  lampblack  the  portion  of  the 
glass  bulb  struck  by  the  beam;  you  see  the  effect :  the 
heat  is  now  absorbed,  the  air  expands,  and  the  thermo- 
metric column  is  forcibly  depressed. 

*  This  was  Melloni's  conclusion ;  but  the  experiments  of  MM.  Provostaye 
and  Desains,  and  of  Mr.  Balfour  Stewart,  prove  that  the  conclusion  is  not 
strictly  correct. 


ACTION   OF  GLASS   FIKE-SCKEENS. 


323 


We  use  glass  fire-screens,  which  allow  the  pleasant 
light  of  the  fire  to  pass,  while  they  cut  off  the  heat ;  the 
reason  is,  that  by  far  the  greater  part  of  the  heat  emitted 
by  a  fire  consists  of  obscure  rays,  to  which  the  glass  is 
opaque.  But  in  no  case  is  there  any  loss.  The  rays  ab- 
sorbed by  the  glass  go  to  warm  the  glass ;  the  motion  of 
the  ethereal  waves  is  transferred  to  the  molecules  of  the 
solid.  But  you  may  be  inclined  to  urge,  that  under  these 
circumstances  the  glass  screen  itself  ought  to  become  a 
source  of  heat,  and  that  therefore  we  ought  to  derive  no 
benefit  from  its  absorption.  The  fact  is  so,  but  the  conclu, 
sion  is  unwarranted.  The  philosophy  of  the  screen  is  this : 

Fig.  88. 


— Let  F  (fig.  88)  be  a  fire  from  which  the  rays  proceed  in 
straight  lines  towards  a  person  at  P.  Before  the  screen  is 
introduced,  each  ray  pursues  its  course  direct  to  P  ;  but 
now  let  a  screen  be  placed  at  s.  The  screen  intercepts  the 
rays  of  heat  and  becomes  warmed  ;  but  instead  of  sending 
on  the  rays  in  their  original  direction  only,  it  emits  them, 
as  a  warm  body,  in  all  directions.  Hence,  it  cannot  re- 
store to  the  person  at  P  all  the  heat  intercepted.  A  por- 
tion of  the  heat  is  restored,  but  by  far  the  greater  part  is 
diverted  from  p,  and  distributed  in  other  directions. 

Where  the  waves  pursue  their  way  unabsorbed,  no  mo- 
tion of  heat  is  imparted,  as  we  have  seen  in  the  case  of  the 


324  LECTUEE  IX. 

air  thermometer.  A  joint  of  meat  might  be  roasted  before 
a  fire,  with  the  air  around  the  joint  as  cold  as  ice.  The  air 
on  high  mountains  may  be  intensely  cold,  while  a  burning 
sun  is  overhead ;  the  solar  rays  which,  striking  on  the  hu- 
man skin,  are  almost  intolerable,  are  incompetent  to  heat 
the  air  sensibly,  and  we  have  only  to  withdraw  into  perfect 
shade  to  feel  the  chill  of  the  atmosphere.  I  never,  on  any 
occasion,  suffered  so  much  from  solar  heat  as  in  descending 
from  the  '  Corridor '  to  the  Grand  Plateau  of  Mont  Blanc, 
on  August  13,  1857  ;  though  hip  deep  in  snow  at  the  time, 
the  sun  blazed  against  me  with  unendurable  power.  Im- 
mersion in  the  shadow  of  the  Dome  du  Goute  at  once 
changed  my  feelings ;  for  here  the  air  was  at  a  freezing 
temperature.  It  was  not,  however,  sensibly  colder  than 
the  air  through  which  the  sunbeams  passed  ;  and  I  suffered, 
not  from  the  contact  of  hot  air,  but  from  the  impact  of 
calorific  rays  which  had  reached  me  through  a  medium  icy 
cold. 

The  beams  of  the  sun  also  penetrate  glass  without  sen- 
sibly heating  it,  and  the  reason  is,  that  having  passed 
through  our  atmosphere,  the  beams  have  been  in  a  great 
measure  deprived  of  those  rays  which  can  be  absorbed  by 
glass.*  I  made  an  experiment  in  a  former  lecture  which 
you  will  now  completely  understand.  I  sent  a  beam  from 
the  electric  lamp  through  a  mass  of  ice  without  melting 
the  substance.  I  had  previously  sifted  the  beam  by  send- 
ing it  through  a  vessel  of  water,  in  which  the  rays  capable 
of  being  absorbed  by  the  ice  were  lodged — and  so  copious- 
ly lodged — that  the  water  was  raised  almost  to  the  boiling 

*  On  d  priori  grounds  I  should  conclude  that  the  obscure  solar  rays 
which  have  succeeded  in  getting  through  our  atmosphere,  must  be  able  to 
penetrate  the  humours  of  the  eye  and  reach  the  retina  :  the  recent  experi- 
ments of  M.  Franz  prove  this.  Their  not  producing  vision  is,  therefore, 
not  due  to  their  absorption  by  the  humours  of  the  eye,  but  to  their  own 
intrinsic  incompetence  to  excite  the  retina. 


KADIATION   THEOUGH   OPAQUE  BODIES.  325 

point  during  the  experiment.  It  is  here  worthy  of  remark 
that  the  liquid  water  and  the  solid  ice  appear  to  be  pervi- 
ous and  impervious  to  the  same  rays ;  the  one  may  be  used 
as  a  sieve  for  the  other ;  a  result  which  indicates  that  the 
quality  of  the  absorption  is  not  influenced  by  the  difference 
of  aggregation  between  solid  and  liquid.  It  is  easy  to 
prove  that  the  beam  which  has  traversed  the  ice  without 
melting  it,  is  really  a  calorific  beam,  by  allowing  it  to  fall 
upon  our  thermo-electric  pile.  Here  is  a  beam  which  has 
passed  through  a  layer  of  water ;  I  permit  it  to  fall  upon 
the  pile,  and  you  instantly  see  its  effect  upon  the  galva- 
nometer, causing  the  needle  to  move  with  energy  to  its 
stops.  Here  is  a  beam  which  has  passed  through  ice,  but 
you  see  that  it  is  equally  competent  to  affect  the  pile  ;  here, 
finally,  is  a  beam  which  has  passed  through  both  water  and 
ice  ;  you  see  it  still  possesses  heating  power.* 

When  the  calorific  rays  are  intercepted,  they,  as  a  gen- 
eral rule,  raise  the  temperature  of  the  body  by  which  they 
are  absorbed;  but  when  the  absorbing  body  is  ice  at  a 
temperature  of  32°  Fahr.,  it  is  impossible  to  raise  its  tem- 
perature. How  then  does  the  heat  absorbed  by  the  ice 
employ  itself  ?  It  produces  internal  liquefaction,  it  takes 
down  the  crystalline  atoms,  and  thus  forms  those  lovely 
liquid  flowers  which  I  showed  you  in  a  former  lecture.f 

We  have  seen  that  transparency  to  light  is  not  at  all  a 
test  of  diathermancy ;  that  a  body  highly  transparent  to 
the  luminous  undulations  may  be  highly  opaque  to  the  non- 
luminous  ones.  I  have  also  given  you  an  example  of  the 
opposite  kind,  and  showed  you  that  a  body  may  be  abso- 
lutely opaque  to  light  and  still,  in  a  considerable  degree, 
transparent  to  heat.  I  set  the  electric  lamp  in  action,  and 

*  Mr.  Faraday  has  fired  gunpowder  by  converging  the  solar  rays  upon  it 
by  a  lens  of  ice. 

f  For  the  bearing  of  these  results  on  air  and  water  bubbles  of  ice,  see 
Appendix  to  Lecture  IX. 


326  LECTTJKE  IX. 

you  see  this  convergent  beam  tracking  itself  through  the 
dust  of  the  room  :  you  see  the  point  of  convergence  of  the 
rays  here,  at  a  distance  of  fifteen  feet  from  the  lamp ;  I 
will  mark  that  point  accurately  by  the  end  of  this  rod. 
Here  is  a  plate  of  rocksalt,  coated  so  thickly  with  soot  that 
the  lighi),  not  only  of  every  gas  lamp  in  this  room,  but  the 
electric  light  itself,  is  cut  off  by  it.  I  interpose  this  plate 
of  smoked  .salt  in  the  path  of  the  beam ;  the  light  is  inter- 
cepted, but  the  rod  enables  me  to  find  with  my  pile  the 
place  where  the  focus  fell.  I  place  the  pile  at  this  focus  : 
you  see  no  beam  falling  on  the  pile,  but  the  violent  action 
of  the  needle  instantly  reveals  to  the  mind's  eye  a  focus  of 
heat  at  the  point  from  which  the  light  has  been  with- 
drawn. 

You  might,  perhaps,  be  disposed  to  think  that  the  heat 
falling  on  the  pile  has  been  absorbed  by  the  soot,  and  then 
radiated  from  it  as  an  independent  source.  Melloni  has  re- 
moved every  objection  of  this  kind  ;  but  none  of  his  exper- 
iments, I  think,  are  more  conclusive,  as  a  refutation  of  the 
objection,  than  that  now  performed  before  you.  For  if 
the  smoked  salt  were  the  source,  the  rays  could  not  con- 
verge here  to  a  focus,  for  the  salt  is  at  this  side  of  the  con- 
verging lens,  and  you  see  when  I  displace  my  pile  a  little 
laterally,  still  keeping  it  turned  towards  the  smoked  salt, 
the  needle  sinks  to  zero. 

The  heat,  moreover,  falling  on  the  pile  is,  as  shown  by 
Melloni,  practically  independent  of  the  position  of  the  plate 
of  rocksalt ;  you  may  cut  off  the  beam  at  a  distance  of  fif- 
teen feet  from  the  pile,  or  at  a  distance  of  one  foot ;  the 
result  is  sensibly  the  same,  which  could  not  be  the  case  if 
the  smoked  salt  itself  were  the  source  of  heat. 

I  make  a  similar  experiment  with  this  black  glass,  and 
the  result,  as  you  see,  is  the  same.  Now  the  glass  reflects 
a  considerable  portion  of  the  light  and  heat  from  the  lamp ; 
if  I  hold  it  a  little  oblique  to  the  beam  you  can  see  the  re- 


PROPORTION   OF   VISIBLE   TO   INVISIBLE   KAYS.         327 

fleeted  portion.  While  the  glass  is  in  this  position  I  will 
coat  it  with  an  opaque  layer  of  lampblack  so  as  to  cause  it 
to  absorb,  not  only  all  the  rays  which  are  now  entering  it, 
but  also  the  portion  which  it  reflects.  What  is  the  result  ? 
Though  the  glass  plate  has  become  the  seat  of  augmented 
absorption,  it  has  ceased  to  affect  the  pile,  the  needle  de- 
scends to  zero,  thus  furnishing  additional  proof  that  the 
rays  which,  in  the  first  place,  acted  upon  the  pile,  came  di- 
rect from  the  lamp,  and  traversed  the  black  glass,  as  light 
traverses  a  transparent  substance. 

Rocksalt  transmits  all  rays,  luminous  and  obscure ; 
alum,  of  the  thickness  already  given,  transmits  only  the  lu- 
minous rays  ;*  hence  the  difference  between  alum  and  rock- 
salt  will  give  the  value  of  the  obscure  radiation.  Tested 
in  this  way,  Melloni  finds  the  following  proportions  of  lu- 
minous to  obscure  rays  for  the  three  sources  mentioned : — 

Source  Luminous  Obscure 

Flame  of  Oil  .  10  90 

Incandescent  Platinum          2  98 

Flame  of  Alcohol    .1  99 

Thus,  of  the  heat  radiated  from  the  flame  of  oil,  90  per 
cent,  is  due  to  the  obscure  rays  ;  of  the  heat  radiated  from 
incandescent  platinum,  98  per  cent,  is  due  to  obscure  rays, 
while  of  the  heat  radiated  from  the  flame  of  alcohol,  fully 
99  per  cent,  is  due  to  the  obscure  radiations. 

*  More  recent  experiments  prove  that  this  is  not  correct. 


APPENDIX    TO    LECTURE    IX. 


EXTRACT  FROM  A  MEMOIR  ON  SOME  PHYSICAL  PROPERTIES  OF 

ICE.* 


I  AVAILED  myself  of  the  fine  sunny  weather  with  which  we 
were  favoured  last  September  and  October,  to  examine  the  effects 
of  solar  heat  upon  ice.  The  experiments  were  made  with  Wen- 
ham  Lake  and  Norway  ice.  Slabs  were  formed  of  the  substance, 
varying  from  one  to  several  inches  in  thickness,  and  these 
were  placed  in  the  path  of  a  beam  rendered  convergent  by  a 
double  convex  lens,  4  inches  in  diameter,  possessing  a  focal  dis- 
tance of  10^  inches.  The  slabs  were  usually  so  placed,  that  the 
focus  of  parallel  rays  fell  within  the  ice.  Having  first  found  the 
position  of  the  focus  in  the  air,  the  lens  was  screened  ;  the  ice  was 
then  placed  in  position,  the  screen  was  removed,  and  the  effect 
was  watched  through  an  ordinary  pocket  lens. 

A  plate  of  ice  an  inch  thick,  with  parallel  sides,  was  first 
examined  :  on  removing  the  screen  the  transparent  mass  was 
crossed  by  the  sunbeams,  and  the  path  of  the  rays  through  it  was 
instantly  studded  by  a  great  number  of  little  luminous  spots,  pro- 
duced at  the  moment,  and  resembling  shining  air-bubbles.  When 
the  beam  was  sent  through  the  edge  of  the  plate,  so  that  it  trav- 
ersed a  considerable  thickness  of  the  ice,  the  path  of  the  beam 
could  be  traced  by  those  brilliant  spots,  as  it  is  by  the  floating 
motes  in  a  dark  room. 

In  lake  ice  the  planes  of  freezing  are  easily  recognized  by  the 
stratified  appearance  which  the  distribution  of  the  air  bubbles 
gives  to  the  substance.  A  cube  was  cut  from  a  perfectly  trans- 

*  Phil.  Trans.  December  1857. 


LIQUID   FLOWEKS   IN  ICE.  329 

parent  portion  of  the  ice,  and  the  solar  beam  was  sent  through 
the  cube  in  three  rectangular  directions  successively.  One  was 
perpendicular  to  the  plane  of  freezing,  and  the  other  two  parallel 
to  it.  The  bright  bubbles  were  formed  in  the  ice  in  all  three 
cases. 

When  the  surfaces  perpendicular  to  the  planes  of  freezing  were 
examined  by  a  lens,  after  exposure  to  the  light,  they  were  found 
to  be  cut  up  by  innumerable  small  parallel  fissures,  with  here  and 
there  minute  spurs  shooting  from  them,  which  gave  the  fissures, 
in  some  cases,  a  feathery  appearance.  When  the  portions  of  the 
ice  traversed  by  the  beam  were  examined  parallel  to  the  surface 
of  freezing,  a  very  beautiful  appearance  revealed  itself.  Allow- 
ing the  light  from  the  window  to  fall  upon  the  ice  at  a  suitable 
incidence,  the  interior  of  the  mass  was  found  filled  with  little 
flower-shaped  figures.  Each  flower  had  six  petals,  and  at  its  cen- 
tre was  a  bright  spot,  which  shone  with  more  than  metallic  bril- 
liancy. The  petals  were  manifestly  composed  of  water,  and  were 
consequently  dim,  their  visibility  depending  on  the  small  differ- 
ence of  refrangibility  between  ice  at  32°  Fahr.  and  water  at  the 
same  temperature. 

For  a  long  time  I  found  the  relation  between  the  planes  of 
these  flowers  and  the  planes  of  freezing  perfectly  constant.  They 
were  always  parallel  to  each  other.  The  developement  of  the 
flowers  was  independent  of  the  direction  in  which  the  beam  trav- 
ersed the  ice.  Hence,  when  an  irregularly  shaped  mass  of  trans- 
parent ice  was  presented  to  me,  by  sending  a  sunbeam  through  it 
I  could  tell  in  an  instant  the  direction  in  which  it  had  been 
frozen. 

Allowing  the  beam  to  enter  the  edge  of  a  plate  of  ice,  and 
causing  the  latter  to  move  at  right  angles  to  the  beam,  so  that  the 
radiant  heat  traversed  different  portions  of  the  ice  in  succession, 
when  the  track  of  the  beam  was  observed  through  an  eye-glass, 
the  ice,  which  a  moment  ago  was  optically  continuous,  was  in- 
stantly starred  by  those  lustrous  little  spots,  and  around  each  of 
them  the  formation  and  growth  of  its  associated  flower  could  be 
distinctly  observed. 

The  maximum  effect  was  confined  to  a  space  of  about  an  inch 
from  the  place  at  which  the  beam  first  struck  the  ice.  In  this 
space  the  absorption,  which  resolved  the  ice  into  liquid  flowers, 


330  APPENDIX  TO   LECTURE   IX. 

for  the  most  part  took  place,  but  I  have~  traced  the  effect  to  a 
depth  of  several  inches  in  large  blocks  of  ice. 

At  a  distance,  however,  from  the  point  of  incidence,  the  spaces 
between  the  flowers  became  greater ;  and  it  was  no  uncommon 
thing  to  see  flowers  developed  in  planes  a  quarter  of  an  inch 
apart,  while  no  change  whatever  was  observed  in  the  ice  between 
these  planes. 

The  pieces  of  ice  experimented  on  appeared  to  be  quite  homo- 
genous, and  their  transparency  was  very  perfect.  Why,  then,  did 
the  substance  yield  at  particular  points  ?  Were  they  weak  points 
of  crystalline  structure,  or  did  the  yielding  depend  upon  the  man- 
ner in  which  the  calorific  waves  impinged  upon  the  molecules  of 
the  body  at  these  points  ?  However  these  and  other  questions 
may  be  answered,  the  experiments  have  an  important  bearing 
upon  the  question  of  absorption.  In  ice  the  absorption  which 
produces  the  flower  is  fitful,  and  not  continuous  ;  and  there  is  no 
reason  to  suppose  that  in  other  solids  the  case  is  not  the  same, 
though  their  constitution  may  not  be  such  as  to  reveal  it.* 

I  have  applied  the  term  '  bubbles '  to  the  little  bright  disks  in 
the  middle  of  the  flowers,  simply  because  they  resembled  the  lit- 
tle air-globules  entrapped  in  the  ice ;  but  whether  they  contained 
air  or  not  could  only  be  decided  by  experiment. 

Pieces  of  ice  were  therefore  prepared,  through  which  the  sun- 
beams were  sent,  so  as  to  develope  the  flowers  in  considerable 
quantity  and  magnitude.  These  pieces  were  then  dipped  into 
warm  water  contained  in  a  glass  vessel,  and  the  effect,  when  the 
melting  reached  the  bright  spots,  was  carefully  observed  through 
a  lens.  The  moment  a  liquid  connection  was  established  between  them 
and  the  atmosphere,  the  TwUbles  suddenly  collapsed,  and  no  trace  of 
air  rose  to  the  surface  of  the  warm  water. 

This  is  the  result  which  ought  to  be  expected.  The  volume 
of  water  at  32°  being  less  than  that  of  ice  at  the  same  tempera- 
ture, the  formation  of  each  flower  ought  to  be  attended  with  the 
formation  of  a  vacuum,  which  disappears  in  the  manner  described 
when  the  ice  surrounding  it  is  melted. 

*  Notwithstanding  the  incomparable  diathermancy  of  the  substance,  M. 
Knoblauch  finds  that  when  plates  of  rocksilt  arc  thick  enough,  they  always 
exhibit  an  elective  absorption.  Effects  like  those  above  described  may 
possibly  be  the  cause  of  this. 


LIQUID  DISKS   EST  ICE.  331 

Similar  experiments  were  made  with  ice,  in  which  true  air- 
bubbles  were  enclosed.  "When  the  melting  liberated  the  air,  the 
bubbles  rose  slowly  through  the  liquid,  and  floated  for  a  time 
upon  its  surface. 

Exposure  for  a  second,  or  even  less,  to  the  action  of  the  sun 
was  sufficient  to  develope  the  flowers  in  the  ice.  The  first  appear- 
ance of  the  central  star  of  light  was  often  accompanied  by  an  au- 
dible clink,  as  if  the  substance  had  been  suddenly  ruptured.  The 
edges  of  the  petals  were  at  the  commencement  definitely  curved ; 
but  when  the  action  was  permitted  to  continue,  and  sometimes  eve'n 
without  this,  when  the  sun  was  strong,  the  edges  of  the  petals  be- 
came serrated,  the  beauty  of  the  figure  being  thereby  augmented. 

Sometimes  a  number  of  elementary  flowers  grouped  together 
to  form  a  thickly-leaved  cluster  resembing  a  rose.  Here  and  there 
also  amid  the  flowers  a  liquid  hexagon  might  be  observed,  but 
euch  were  of  rare  occurrence. 

The  act  of  crystalline  dissection,  if  I  may  use  the  term,  thus 
performed  by  the  solar  beams,  is  manifestly  determined  by  the 
manner  in  which  the  crystalline  forces  have  arranged  the  mole- 
cules. By  the  abstraction  of  heat  the  molecules  are  enabled  to 
fcuild  themselves  together,  by  the  introduction  of  heat  this  archi- 
tecture is  taken  down.  The  perfect  symmetry  of  the  flowers,  from 
ivhieh  there  is  no  deviation,  argues  a  similar  symmetry  in  the 
iiiolecular  architecture  ;  and  hence,  as  optical  phenomena  depend 
upon  the  molecular  arrangement,  we  might  pronounce  with  per- 
lect  certainty  from  the  foregoing  experiments,  that  ice  is,  what 
Sir  David  Brewster  long  ago  proved  it  to  be,  optically  speaking, 
uuiaxal,  the  axis  being  perpendicular  to  the  surface  of  freezing. 

§n. 

On  September  25,  while  examining  a  perfectly  transparent 
piece  of  Norway  ice,  which  had  not  been  traversed  by  the  con- 
densed sunbeams,  I  found  the  interior  of  the  mass  crowded  with 
parallel  liquid  disks,  varying  in  diameter  from  the  tenth  to  the 
hundredth  of  an  inch.  These  disks  were  so  thin,  that  when  looked 
at  in  section  they  were  reduced  to  the  finest  lines.  They  had  the 
exact  appearance  of  the  circular  spots  of  oily  scum  which  float  on 
the  surface  of  mutton  broth,  and  in  the  pieces  of  ice  first  exam- 
ined they  always  lay  in  tte.  planes  of  freezing. 


332  APPENDIX  TO   LECTURE  IX. 

As  time  progressed,  this  internal  disintegration  of  the  ice  ap- 
peared to  become  more  pronounced,  so  that  some  pieces  of  Nor- 
way ice  examined  in  the  middle  of  November  appeared  to  be  re- 
duced to  a  congeries  of  water-cells  entangled  in  a  skeleton  of  ice. 
The  effect  of  this  was  rendered  manifest  to  the  hand  on  sawing  a 
block  of  this  ice,  by  the  facility  with  which  the  saw  went 
through  it. 

There  seems  to  be  no  such  thing  as  absolute  homogeneity  in 
nature.  Change  commences  at  distinct  centres,  instead  of  being 
uniformly  and  continuously  distributed,  and  in  the  most  appar- 
ently homogeneous  substance  we  should  discover  defects,  if  our 
means  of  observation  were  fine  enough.  The  above  observations 
show  that  some  portions  of  a  mass  of  ice  melt  more  readily  than 
others.  The  melting  temperature  of  the  substance  is  set  down  at 
32°  Fahr.,  but  the  absence  of  perfect  homogeneity,  whether  from 
difference  of  crystalline  texture  or  some  other  cause,  makes  the 
melting  temperature  oscillate  to  a  slight  extent  on  both  sides  of 
the  ordinary  standard.  Let  this  limit,  expressed  in  parts  of  a  de- 
gree, be  t.  Some  parts  of  a  block  of  ice  will  melt  at  a  tempera- 
ture of  32 — £,  while  others  require  a  temperature  of  32  4- 1 :  the 
consequence  is,  that  such  a  block  raised  to  the  temperature  of  32°, 
will  have  some  of  its  parts  liquid,  and  others  solid. 

When  a  mass  exhibiting  the  water-disks  was  examined  by  a 
concentrated  sunbeam,  the  six-leaved  flowers  before  referred  to 

were  always  formed  in  tlie  planes  of  the  disks. 

******* 

§  HI. 

What  has  been  already  said  will  prepare  us  for  the  considera- 
tion of  an  associated  class  of  phenomena  of  great  physical  inter- 
est. The  larger  masses  of  ice  which  I  examined  exhibited  layers, 
in  which  bubbles  of  air  were  collected  in  unuusal  quantity,  mark- 
ing, no  doubt,  the  limits  of  successive  acts  of  freezing.  These 
bubbles  were  usually  elongated.  Between  two  such  beds  of  bub- 
bles a  clear  stratum  of  ice  intervened ;  and  a  clear  surface  layer, 
which,  from  its  appearance,  seemed  to  have  suffered  more  from, 
external  influences  than  the  rest  of  the  ice,  was  associated  with 
each  block.  In  this  superficial  portion  I  observed  detached  air- 
bubbles  irregularly  distributed,  and  associated  with  each  vesicle 


INTERNAL   LIQUEFACTION.  333 

of  air,  a  bleb  of  water  which  had  the  appearance  of  a  drop  of  clear 
oil  within  the  solid.     The  adjacent  figure  will  give  a  notion  of 
these  composite  cavities  :  the  unshaded  cir- 
cle represents  the  air-bubble,  and  the  shaded 
space  adjacent,  the  water. 

When  the  quantity  of  water  was  suffi- 
ciently large,  which  was  usually  the  case,  on 
turning  the  ice  round,  the  bubble  shifted  its 
position,  rising  always  at  the  top  of  the  bleb 
of  water.     Sometimes,  however,  the  cell  was 
very  flat,  and  the  air  was  then  quite  surrounded  by  the  liquid. 
These  composite  cells  often  occurred  in  pellucid  ice,  which  showed 
inwardly  no  other  sign  of  disintegration. 

This  is  manifestly  the  same  phenomenon  as  that  which  struck 
M.  Agassiz  so  forcibly  during  his  ealier  investigations  on  the  gla- 
cier of  the  Aar.  The  same  appearances  have  been  described  by 
the  Messrs.  Schlagintweit,  and  finally  attention  has  been  forcibly 
drawn  to  the  subject  in  a  recent  paper  by  Mr.  Huxley,  published 
in  the  '  Philosophical  Magazine.'  * 

The  only  explanation  of  this  phenomenon  hitherto  given,  and 
adopted  apparently  without  hesitation,  is  that  of  M.  Agassiz  and 
the  Messrs.  Schlagintweit.  These  observers  attribute  the  phe- 
nomenon to  the  diathermancy  of  the  ice,  which  permits  the  radi- 
ant heat  to  pass  through  the  substance,  to  heat  the  bubbles  of  air, 
and  cause  them  to  melt  the  surrounding  ice.t 

The  apparent  simplicity  of  this  explanation  contributed  to 
ensure  its  general  acceptance ;  and  yet  I  think  a  little  reflection 
will  show  that  the  hypothesis,  simple  as  it  may  appear,  is  attend- 
ed with  grave  difficulties. 

For  the  sake  of  distinctness  I  will  here  refer  to  a  most  interest- 
ing fact,  observed  first  by  M.  Agassiz,  and  afterwards  by  the  Messrs. 
Schlagintweit.  In  the  *  Systerne  Glaciaire '  it  is  described  in  these 

*  October,  1857. 

f  II  est  evident  pour  quiconque  a  suivi  leprogres  de  la  physique  mo- 
derne,  que  ce  phenomene  est  du  uniquement  a  la  diathermaneite  de  la 
glace  (Agassiz,  Systeme,  p.  157). 

Das  Wasser  ist  dadurch  enstanden  dass  die  Luft  Warmestrahlen  absor- 
birte,  welche  das  Eis  als  diathermaner  Korper  durchliess  (Schlagintweit, 
Untersuchungen,  S.  17). 


334:  APPENDIX  TO  LECTURE  IX. 

words :  '  I  ought  also  to  mention  a  singular  property  of  those 
air-bubbles,  which  at  first  struck  us  forcibly,  but  which  has  since 
recevied  a  very  satisfactory  explanation.  When  a  fragment  con- 
taining air-bubbles  is  exposed  to  the  action  of  the  sun,  the  bub- 
bles augment  insensibly.  Soon,  in  proportion  as  they  enlarge,  a 
transparent  drop  shows  itself  at  some  point  of  the  bubble.  This 
drop,  in  enlarging,  contributes  on  its  part  to  the  enlargement  of 
the  cavity,  and  following  its  progress  a  little,  it  finishes  by  pre- 
dominating over  the  bubble  of  air.  The  latter  then  swims  in  the 
midst  of  a  zone  of  water,  and  tends  incessantly  to  reach  the  most 
elevated  point,  at  least  if  the  flatness  of  the  cavity  does  not  hin- 
der it.' 

The  satisfactory  explanation  here  spoken  of  is  that  already 
mentioned  :  let  us  now  endeavour  to  follow  the  hypothesis  to  its 
consequences. 

Comparing  equal  weights  of  both  substances,  the  specific  heat 
of  water  being  1,  that  of  air  is  0'25.  Hence  to  raise  a  pound  of 
water  one  degree  of  temperature,  a  pound  of  air  would  have  to 
lose  four  degrees. 

Let  us  next  compare  equal  volumes  of  the  substances.  The 
specific  gravity  of  water  being  1,  that  of  the  air  is  T^_  •  hence  a 
pound  of  air  is  770  times  the  volume  of  a  pound  of  water ;  and 
hence,  for  a  quantity  of  air  to  raise  its  own  volume  of  water  one 
degree,  it  must  part  with  770x4,  or  3,080  degrees  of  temperature. 

Now  the  latent  heat  of  water  is  142-6°  Fahr.,  hence  the  quan- 
tity of  heat  required  to  melt  a  certain  weight  of  ice  is  142-6  times 
the  quantity  required  to  raise  the  same  weight  of  water  one  de- 
gree in  temperature  ;  hence,  a  measure  of  air,  in  order  to  reduce 
its  own  volume  of  ice  to  the  liquid  condition,  must  lose  3,080  x 
142-6,  or  439,208  degrees  of  temperature. 

This,  then,  gives  us  an  idea  of  the  amount  of  heat  which,  ac- 
cording to  the  above  hypothesis,  is  absorbed  by  the  bubble  and 
communicated  to  the  ice  during  the  time  occupied  in  melting  a 
quantity  of  the  latter  equal  in  volume  to  the  bubble,  which  time 
is  stated  to  be  brief ;  that  is  to  say  the  quantity  of  heat  supposed 
to  be  absorbed  by  the  air  would,  if  it  had  not  been  communicated 
to  the  ice,  have  been  sufficient  to  raise  the  bubble  itself  to  a  tem- 
perature 160  times  that  of  fused  cast  iron.  Had  air  this  power 
of  absorption,  it  might  be  attended  with  inconvenient  conse- 


ASSOCIATED   BUBBLES   OF   AIR  AND   WATEK.  335 

quences  to  the  denizens  of  the  earth  ;  for  we  should  dwell  at  the 
bottom  of  an  atmospheric  ocean,  the  upper  strata  of  which  would 
effectually  arrest  all  calorific  radiation. 

It  is  established  by  the  experiments  of  Delaroche  and  Mel- 
loni,*  that  a  calorific  beam,  emerging  from  any  medium  which  it 
has  traversed  for  any  distance,  possesses,  in  an  exalted  degree, 
the  power  of  passing  through  an  additional  length  of  the  same 
substance.  Absorption  takes  place,  for  the  most  part,  in  the  por- 
tion of  the  medium  first  traversed  by  the  rays.  In  the  case  of  a 
plate  of  glass,  for  example,  17^  per  cent,  of  the  heat  proceeding 
from  a  lamp,  is  absorbed  in  the  first  fifth  of  a  millimetre  ;  where- 
as, after  the  rays  have  passed  through  6  millimetres  of  the  sub- 
stance, an  additional  distance  of  2  millimetres  absorbs  less  than  2 
per  cent,  of  the  rays  thus  transmitted.  Supposing  the  rays  to 
have  passed  through  a  plate  25  millimetres,  or  an  inch,  in  thick- 
ness, there  is  no  doubt  that  the  heat  emerging  from  such  a  plate 
would  pass  through  a  second  layer  of  glass,  1  millimetre  thick, 
without  suffering  any  measurable  absorption.  For  an  incompar- 
ably stronger  reason,  the  quantity  of  solar  heat  absorbed  by  a 
bubble  of  air  at  the  earth's  surface,  after  the  rays  have  traversed 
the  whole  thickness  of  our  atmosphere,  and  been  sifted  in  their 
passage  through  it,  mut  be  wholly  inappreciable. 

Such,  if  I  mistake  not,  are  the  properties  of  radiant  heat  which 
modern  physics  have  revealed ;  and  I  think  they  render  it  evident 
that  the  hypothesis  of  M.  Agassiz  and  the  Messrs.  Schlagintweit 
was  accepted  without  due  regard  to  its  consequences. 

*  *  *  #  *  *  * 


§  IV. 

But  the  question  still  remains,  how  are  the  water-chambers 
produced  within  the  ice  ?  ...  One  simple  test  will,  I  think,  de- 
cide the  question  whether  the  liquid  is,  or  is  not,  the  product  of 
melted  ice.  If  it  be,  its  volume  must  be  less  than  that  of  the  ice 
which  produced  it,  and  the  bubble  associated  with  the  water  must 
l)e  composed  of  rarefied  air.  Hence,  if  on  establishing  a  liquid  con- 
nection between  this  bubble  and  the  atmosphere  a  diminution  of 

*  La  Thermoclirose,  p.  202. 


330  APPENDIX  TO   LECTURE   IX. 

volume  be  observed,  this  will  indicate  that  the  water  has  been 
produced  by  the  melting  of  the  ice. 

From  a  block  of  Norway  ice,  containing  such  compound  bub- 
bles, I  cut  a  prism,  and  immersing  it  in  warm  water,  contained  in 
a  glass  vessel,  I  carefully  watched  through  the  side  of  the  vessel 
the  effect  of  the  melting  upon  the  bubbles.  They  invariably  shrunk 
in  volume  at  the  moment  the  surrounding  ice  was  melted,  and  the 
diminished  globules  of  air  rose  to  the  surface  of  the  water.  I 
then  arranged  matters  so  that  the  wall  of  the  cavity  might  be 
melted  away  underneath,  without  permitting  the  bubble  of  air  at 
the  top  to  escape.  At  the  moment  the  melting  reached  the  cavity 
the  air-bubbles  instantly  collapsed  to  a  sphere  possessing,  in  some 
cases,  far  less  than  the  hundredth  part  of  its  original  volume. 
The  experiments  were  repeated  with  several  distinct  masses  of  ice, 
and  always  with  the  same  result.  I  think,  therefore,  it  may  be 
regarded  as  certain  that  the  liquid  cells  are  the  product  of  melted 
ice.* 

Considering  the  manner  in  which  ice  imported  into  this  coun- 
try is  protected  from  the  solar  rays,  I  think  we  must  infer  that  in 
the  specimens  examined  by  me,  tlie  ice  in  contact  with  the  bubble 
has  ~been  melted  ly  heat,  which  has  teen  conducted  through  the  sub- 
stance without  visible  prejudice  to  its  solidity. . 

Paradoxical  as  this  may  appear,  I  think  it  is  no  more  than 
might  reasonably  be  expected  from  d  priori  considerations.  The 
heat  of  a  body  is  referred,  at  the  present  day,  to  a  motion  of  its 
particles.  When  this  motion  reaches  an  intensity  sufficient  to 
liberate  the  particles  of  a  solid  from  their  mutual  attractions,  the 
body  passes  into  the  liquid  condition.  Now,  as  regards  the 
amount  of  motion  necessary  to  produce  this  liberty  of  liquidity, 
the  particles  at  the  surface  of  a  mass  of  ice  must  be  very  different- 
ly circumstanced  from  those  in  the  interior,  which  are  influenced 
and  controlled  on  every  side  by  other  particles.  But  if  we  sup- 
pose a  cavity  to  exist  within  the  mass,  the  particles  bounding  that 
cavity  will  be  in  a  state  resembling  that  of  the  particles  at  the 
surface ;  and  by  the  removal  of  all  opposing  action  on  one  side, 
the  molecules  may  be  liberated  by  a  force  which  the  surrounding 
mass  has  transmitted  without  prejudice  to  its  solidity.  Suppos- 

*  This  of  course  refers  only  to  the  lake  ice  examined  as  described. 


LIQUEFACTION  BY   CONDUCTION   THROUGH   ICE.        337 

ing,  for  example,  that  solidity  is  limited  by  molecular  vibrations 
of  a  certain  amplitude,  those  at  the  surface  of  the  internal  cavity 
may  exceed  this  limit,  while  those  between  the  cavity  and  the  ex- 
ternal surface  of  the  ice  may,  by  their  reciprocal  actions,  be  pre- 
served within  it,  just  as  the  terminal  member  of  a  series  of  elastic 
balls  is  detached  by  a  force  which  has  been  transmitted  by  the 
other  members  of  the  series  without  visible  separation.* 

Where,  however,  experiment  is  within  reach  we  ought  not  to 
trust  to  speculation ;  and  I  was  particularly  anxious  to  obtain  an 
unequivocal  reply  to  the  question  whether  an  interior  portion  of 
a  mass  of  ice  could  be  melted  by  heat  which  had  passed  through 
the  substance  by  the  process  of  conduction.  A  piece  of  Norway 
ice,  containing  a  great  number  of  the  liquid  disks  already  de- 
scribed, and  several  cells  of  air  and  water,  was  enveloped  in  tin- 
foil -and  placed  in  a  mixture  of  pounded  ice  and  salt.  A  few 
minutes  sufficed  to  freeze  the  disks  to  thin  dusky  circles,  which 
appeared,  in  some  cases,  to  be  formed  of  concentric  rings,  and 
reminded  me  of  the  sections  of  certain  agates.  Looked  at  side- 
ways, these  disks  were  no  thicker  than  a  fine  line.  The  water- 
cells  were  also  frozen,  and  the  associated  air-bubbles  were  greatly 
diminished  in  size.  I  placed  tbfc  mass  of  ice  between  me  and  a 
gas-light,  and  observed  it  through  a  lens :  after  some  time  the 
disks  and  water-cells  showed  signs  of  breaking  up.  The  rings  of 
the  disks  disappeared ;  the  contents  seemed  to  aggregate  so  as  to 
form  larger  liquid  spots,  and  finally,  some  of  them  were  reduced 
to  clear  transparent  disks  as  before. 

But  an  objection  to  this  experiment  is,  that  the  ice  may  have 
been  liquefied  by  the  radiation  from  the  lamp,  and  I  have  experi- 
ments to  describe  which  will  show  the  justice  of  this  objection. 
A  rectangular  slab,  1  inch  thick,  3  inches  long,  and  2  wide,  was 
therefore  taken  from  a  mass  of  Norway  ice,  in  which  the  associat- 
ed air  and  water-cells  were  very  distinct.  I  enveloped  it  in  tin- 
foil, and  placed  it  in  a  freezing  mixture.  In  about  ten  minutes 
the  water-blebs  were  completely  frozen  within  the  mass.  It  was 
immediately  placed  in  a  dark  room,  where  no  radiant  heat  could 
possibly  affect  it,  and  examined  every  quarter  of  an  hour.  The 
dim  frozen  spots  gradually  broke  up  into  little  water  parcels,  and 

*  Of  course  I  intend  this  to  help  the  conception  merely. 
15 


338  APPENDIX  TO  LECTTJKE  IX. 

in  two  hours  the  water-blebs  were  perfectly  restored  in  the  centre 
of  the  slab  of  ice.  When  last  examined,  this  plate  was  half  an 
inch  thick,  and  the  drops  of  liquid  were  seen  right  at  its  centre. 

A  second  piece,  similarly  frozen  and  wrapped  up  in  flannel, 
showed  the  same  deportment.  In  an  hour  and  a  half  the  frozen 
water  surrounding  the  air-bubbles  was  restored  to  its  liquid  con- 
dition. Hence  no  doubt  can  remain  as  to  the  possibility  of  effect- 
ing liquefaction  in  the  interior  of  a  mass  of  ice,  by  heat  which 
has  passed  by  conduction  through  the  substance  without  melting  it. 

I  have  already  referred  to  the  formation  of  the  liquid  cavities 
observed  by  M.  Agassiz,  when  glacier  ice  was  exposed  to  the  sun. 
The  same  effect  may  be  produced  by  exposure  to  a  glowing  coal 
fire.  On  the  21st  and  22nd  of  November,  I  thus  exposed  plates 
of  clear  Wenham  Lake  ice,  which  contained  some  scattered  air- 
bubbles.  At  first  the  bubbles  were  sharply  rounded,  and  without 
any  trace  of  water.  Soon,  however,  those  near  the  surface,  on 
which  the  radiant  heat  fell,  appeared  encircled  by  a  liquid  ring, 
r^^v.  which  expanded  and  finally  became  crimped  at  its  border, 
r  O j  as  shown  in  the  adjacent  figure.  The  crimping  became 
^-^  more  pronounced  as  the  action  was  permitted  to  continue.* 

A  second  plate,  crowded  with  bubbles,  was  held  as  near  to  the 
fire  as  the  hand  could  bear.  On  withdrawing  it,  and  examining 
it  through  a  pocket  lens,  the  appearance  was  perfectly  beautiful. 
In  many  cases  the  bubbles  appeared  to  be  surrounded  by  a  series 
of  concentric  rings,  the  outer  ring  surrounding  all  the  others  like 
a  crimped  frill. 

I  could  not  obtain  these  effects  by  placing  the  ice  in  contact 
with  a  plate  of  metal  obscurely  heated,t  nor  by  the  radiation  from 
an  obscure  source.  Indeed  ice,  as  before  remarked,  is  impervious 
to  radiant  heat  from  such  a  source.^  The  rays  from  a  common 

*  The  blebs  observed  in  glacier  ice  also  exhibit  this  form :  see  fig.  8, 
plate  6,  of  the  Atlas  to  the  '  Systeme  Glaciaire.'  In  fig.  13  we  have  also  a 
close  resemblance  of  the  flower-shaped  figures  produced  by  radiant  heat  in 
lake  ice. 

f  To  develope  water-cavities  within  ice  a  considerable  time  is  necessary ; 
more  time,  indeed,  than  was  sufficient  to  melt  the  entire  pieces  of  ice  made 
use  of  in  these  contact  experiments. 

\  Hence  the  soundness  of  the  ice  under  the  moraines ;  the  sun's  rays 
are  converted  into  obscure  heat  by  the  overlying  debris ;  this  only  affects  a 


EEGELATION.  339 

fire  also  are  wholly  absorbed  near  the  surface  upon  which  they 
strike,  and  hence  the  described  internal  liquefaction  was  confined 
to  a  thin  layer  close  to  this  surface. 

But  not  only  does  liquefaction  occur  in  connection  with  the 
bubbles,  but  the  '  flowers,'  already  described  as  produced  by  the 
solar  beams,  start  by  hundreds  into  existence,  when  a  slab  of 
transparent  ice  is  placed  before  a  glowing  coal  fire.  They,  how- 
ever, are  also  confined  to  a  thin  stratum  of  the  substance  close  to 
the  surface  of  incidence.  In  the  experiments  made  in  this  way, 
the  central  stars  of  the  flowers  were  often  bounded  by  sinuous 
lines  of  great  beauty. 

The  foregoing  considerations  show  that  liquefaction  takes 
place  at  the  surface  of  a  mass  of  ice  at  a  lower  temperature  than 
that  required  to  liquefy  the  interior  of  the  solid.  At  the  surface 
the  temperature  32°  produces  a  vibration,  to  produce  which,  with- 
in the  ice,  would  necessitate  a  temperature  of  32°  +  x ;  the  incre- 
ment #  being  the  additional  temperature  necessary  to  overcome 
the  resistance  to  liquefaction,  arising  from  the  action  of  the  mole- 
cules upon  each  other. 

Now  let  us  suppose  two  pieces  of  ice  at  32°,  with  moistened 
surfaces,  to  be  brought  into  contact  with  each  other,  we  thereby 
virtually  transfer  the  touching  portions  of  these  pieces  from  the  sur- 
face to  the  interior,  where  32 +x  is  the  melting  temperature. 
Liquefaction  will  therefore  be  arrested  at  those  surfaces.  Before 
being  brought  together,  the  surfaces  had  the  motion  of  liquidity, 
but  the  interior  of  the  ice  has  not  this  motion ;  and  as  equilibrium 
will  soon  set  in  between  the  masses  on  each  side  of  the  liquid  film 
and  the  film  itself,  the  film  will  be  reduced  to  a  state  of  motion 
inconsistent  with  liquidity.  In  other  words,  it  will  ~be  frozen,  and 
will  cement  the  two  surfaces  of  ice  between  which  it  is  enclosed* 

If  I  am  right  here,  the  importance  of  the  physical  principles 

layer  of  infinitesimal  depth,  and  cannot  produce  the  disintegration  of  the 
deeper  ice,  as  the  direct  sunbeams  can. 

*  It  is  here  implied  that  the  contact  of  the  moist  surfaces  must  be  so 
perfect,  or,  in  other  words,  the  liquid  film  between  them  must  be  so  thin, 
as  to  enable  the  molecules  to  act  upon  each  other  across  it.  The  extreme 
tenuity  of  the  film  may  be  inferred  from  this.  A  thick  plate  of  water 
within  the  ice  Avould  facilitate  rather  than  retard  liquefaction. 


340  APPENDIX  TO  LECTUKE  IX. 

involved  arc  sufficiently  manifest :  if  I  am  wrong,  I  hope  I  have 
so  expressed  myself  as  to  render  the  detection  of  my  error  easy. 
Eight  or  wrong,  my  aim  has  been  to  give  as  explicit  utterance  to 
my  meaning  as  the  subject  will  admit  of. 

§  V. 

Mr.  Faraday's  experiments  on  the  freezing  together  of  pieces  of 
ice  at  32°  Fahr.-,  and  all  of  those  recounted  in  the  paper  published 
by  Mr.  Huxley  and  myself,  find  their  explanation  in  the  principles 
here  laid  down.  The  conversion  of  snow  into  neve,  and  of  neve 
into  glacier,  is  perhaps  the  grandest  illustration  of  the  same  prin- 
ciple. It  has  been,  however,  suggested  to  me  that  the  sticking 
together  of  two  pieces  of  ice  may  be  an  act  of  cohesion,  similar  to 
that  which  enables  pieces  of  wetted  glass,  and  other  similar 
bodies,  to  stick  together.  This  is  not  the  case.  There  is  no  slid- 
ing motion  possible  to  the  ice.  When  contact  is  broken,  it  breaks 
with  the  snap  due  to  the  rupture  of  a  solid.  Glass  and  ice  cannot 
be  made  to  stick  thus  together,  neither  can  glass  and  glass,  nor 
alum  and  alum,  nor  nitre  and  nitre,  at  common  temperatures.  I 
have,  moreover,  placed  pieces  of  ice  together  over  night  and  found 
them  in  the  morning  so  rigidly  frozen  together  that  when  I  sought 
to  separate  them,  the  surface  of  fracture  passed  through  one  of 
them  in  preference  to  taking  the  surface  of  regelation.  Many 
sagacious  persons  have  also  suggested  to  me  that  the  ice  trans- 
ported to  this  country  from  Norway  and  Wenham  Lake  may  pos- 
sibly retain  a  residue  of  its  cold,  sufficient  to  freeze  a  thin  film 
enclosed  between  two  pieces  of  the  substance.  But  the  facts  al- 
ready adverted  to  are  a  sufficient  reply  to  this  surmise.  The  ice 
experimented  on  cannot  be  regarded  as  a  magazine  of  cold,  became 
of  liquid  water  exist  within  it. 


LECTURE    X. 

[March  27,  1862.] 

ABSORPTION  OF  HEAT  BY  GASEOUS  MATTER — APPARATUS  EMPLOYED — EARLT 
DIFFICULTIES DIATHERMANCY  OF  AIR  AND  OF  THE  TRANSPARENT  ELE- 
MENTARY GASES — ATHERMANCY  (OPACITY)  OF  OLEFIANT  GAS  AND  OF  THE 
COMPOUND  GASES — ABSORPTION  OF  RADIANT  HEAT  BY  VAPOURS — RADIA- 
TION OF  HEAT  BY  GASES RECIPROCITY  OF  RADIATION  AND  ABSORPTION 

— INFLUENCE    OF    MOLECULAR    CONSTITUTION    ON    THE    PASSAGE    OF    RA- 
DIANT  HEAT. 

IN  our  last  lecture  we  examined  the  diathermancy,  or 
transparency  to  heat,  of  solid  and  liquid  bodies  ;  and  we 
then  learned,  that  closely  as  the  atoms  of  such  bodies  are 
packed  together,  the  interstitial  spaces  between  the  atoms 
afford,  in  many  cases,  free  play  and  passage  to  the  ethereal 
undulations,  which  were  transmitted  without  sensible  hin- 
drance among  the  atoms.  In  other  cases,  however,  we 
found  that  the  molecules  stopped  the  waves  of  heat  which 
impinged  upon  them ;  but  that  in  so  doing,  they  them- 
selves became  centres  of  oscillation.  Thus  we  learned  that 
while  perfectly  diathermic  bodies  allowed  the  waves  of 
heat  to  pass  through  them  without  suffering  any  change  of 
temperature,  Tfchose  bodies  which  stopped  the  calorific  flux 
became  heated  by  the  absorption.  Through  ice,  itself,  we 
sent  a  powerful  calorific  beam ;  but  as  the  beam  was  of 
such  a  quality  as  not  to  be  intercepted  by  the  ice,  it  passed 
through  this  highly  sensitive  substance  without  melting  it. 
We  have  now  to  deal  with  gaseous  bodies ;  and  here  the 
interatomic  spaces  are  so  vastly  augmented,  the  molecules 


34:2  LECTURE   X. 

are  so  completely  released  from  all  mutual  entanglement, 
that  we  should  be  almost  justified  in  concluding  that  gases 
and  vapours  furnish  a  perfectly  open  door  for  the  passage 
of  the  calorific  waves.  This,  indeed,  until  quite  recently, 
was  the  universal  belief,  and  the  conclusion  was  verified  by 
such  experiments  as  had  been  made  on  atmospheric  -air, 
which  was  found  to  give  no  evidence  of  absorption. 

But  each  succeeding  year  augments  our  experimental 
powers  ;  our  predecessors  were  often  obliged  to  fight  with 
flints,  where  we  may  use  swords,  and  hence  the  conflict 
with  Nature  is  not  decided  by  their  discomfiture.  Let  us, 
then,  test  once  more  the  diathermancy  of  atmospheric  air. 
We  may  make  a  preliminary  essay  in  the  following  way  : 
I  have  here  a  hollow  tin  cylinder  A  B  (fig.  89),  4  feet  long, 
and  nearly  3  inches  in  diameter,  through  which  we  may 
send  our  calorific  rays.  We  must,  however,  be  able  to  com- 
pare the  passage  of  the  rays  through  the  air^  with  their 
passage  through  a  vacuum,  and  hence  we  must  have  some 
means  of  stopping  the  ends  of  our  cylinder,  so  as  to  be 
able  to  exhaust  it.  Here  we  encounter  our  first  experi- 
mental difficulty.  As  a  general  rule  obscure  heat  is  more 
greedily  absorbed  than  luminous  heat,  and  as  our  object  is 
to  make  the  absorption  of  a  highly  diathermic  body  sensi- 
ble, we  are  most  likely  to  effect  this  object  by  employing 
obscure  heat. 

Our  tube,  therefore,  must  be  stopped  by  a  substance 
which  permits  of  the  free  passage  of  such  heat.  Shall  we 
use  glass  for  the  purpose  ?  An  inspection  of  the  table  at 
page  311  shows  us,  that  for  such  rays  plates  of  glass  would 
be  perfectly  opaque  ;  we  might  as  well  stop  our  tube  with 
plates  of  metal.  Observe  here  how  an  investigator's  results 
are  turned  to  account  by  his  successors.  From  one  experi- 
ment buds  another,  and  science  grows  by  the  continual  deg- 
radation of  ends  to  means.  Had  not  Melloni  discovered 
the  diathermic  properties  of  rocksalt,  we  should  now  be  ut- 


FIRST   EXPERIMENTS   WITH   GASES. 


343 


terly  at  a  loss.  For  a  time,  however,  I  was  extremely  ham- 
pered by  the  difficulty  of  obtaining  plates  of  salt  sufficiently 
large  and  pure  to  stop  the  ends  of  my  tube.  But  a  scientific 
worker  does  not  long  lack  help,  and,  thanks  to  such  friend- 
ly aid,  I  have  here  plates  of  this  precious  substance  which, 
by  means  of  these  caps,  I  can  screw  air-tight  on  to  the  ends 

Fig.  89. 


of  my  cylinder.*    You  observe  two  stopcocks  attached  to 
the  cylinder ;  this  one,  c,  is  connected  with  an  air-pump,  by 

*  At  a  time  when  I  was  greatly  in  need  of  a  supply  of  rocksalt,  I  stated 
iny  wants  in  the  *  Philosophical  Magazine,'  and  met  with  an  immediate  re- 
sponse from  Sir  John  Herschel.  He  sent  me  a  block  of  salt,  accompanied 
by  a  note,  from  which,  as  it  refers  to  the  purpose  for  which  the  salt  was 
originally  designed,  I  will  make  an  extract.  I  have  not  yet  been  able  to 
examine  the  extremely  remarkable  point  to  which  the  eminent  writer  di- 
rects my  attention.  I  am  also  greatly  indebted  to  Dr.  Szabo,  the  Hunga- 
rian Commissioner  to  the  International  Exhibition,  by  whom  I  have  been 
lately  raised  to  comparative  opulence,  as  regards  the  possession  of  rocksalt. 
To  the  Messrs.  Fletcher,  of  Northwich,  and  to  Mr.  Corbett,  of  Broms- 
grove,  my  best  thanks  are  also  due  for  their  obliging  kindness. 

Here  follows  the  extract  from  Sir  J.  Herschcl's  note  : — '  After  the  publi- 
cation of  my  paper  in  the  Phil.  Trans.,  1840,  I  was  very  desirous  to  disen- 
gage myself  from  the  influence  of  glass  prisms  and  lenses,  and  ascertain,  if 
possible,  whether  in  reality  my  insulated  heat  spots  /3  7  8  e  in  the  spectrum 


LECTURE   X. 

which  the  tube  can  be  exhausted ;  while  through  this  other 
one,  c',  I  can  allow  air  or  any  other  gas  to  enter  the  tube. 

At  one  end  of  the  cylinder  I  place  this  Leslie's  cube  c, 
containing  boiling  water ;  and  which  is  coated  with  lamp- 
black, to  augment  its  power  of  radiation.  At  the  other  end 
of  the  cylinder  stands  our  thermo-electric  pile,  from  which 
wires  lead  to  the  galvanometer.  Between  the  end  of  the 
cylinder  and  the  source  of  heat  I  have  introduced  a  tin 
screen,  T,  which,  when  withdrawn,  will  allow  the  calorific 
rays  to  pass  through  the  tube  to  the  pile.  We  first  exhaust 
the  cylinder,  then  draw  the  screen  a  little  aside,  and  now 
the  rays  are  traversing  a  vacuum  and  falling  upon  the  pile. 
The  tin  screen,  you  observe,  is  only  partially  withdrawn, 
and  the  steady  deflection  produced  by  the  heat  at  present 
transmitted  is  30  degrees. 

Let  us  now  admit  dry  air  :  I  can  do  so  by  means  of  the 
cock  c',  from  which  a  piece  of  flexible  tubing  leads  to  the 
bent  tubes  F,  tr',  the  use  of  which  I  will  now  explain ;  u  is 
filled  with  fragments  of  pumice  stone  moistened  with  a  so- 
lution of  caustic  potash ;  it  is  destined  to  withdraw  what- 

were  of  solar  or  terrestial  origin.  Rocksalt  was  the  obvious  resource,  and 
after  many  and  fruitless  endeavours  to  obtain  sufficiently  large  and  pure 
specimens,  the  late  Dr.  Somerville  was  so  good  as  to  send  me  (as  I  under- 
stood from  a  friend  in  Cheshire)  the  very  fine  block  which  I  now  forward. 
It  is,  however,  much  cracked,  but  I  have  no  doubt  pieces  large  enough  for 
lenses  and  prisms  (especially  if  cemented  together)  might  be  got  from  it. 

1  But  I  was  not  prepared  for  the  working  of  it — evidently  a  very  delicate 
and  difficult  process,  (I  proposed  to  dissolve  off  the  corners,  &c.,  and,  as  it 
were,  lick  it  into  shape)  and  though  I  have  never  quite  lost  sight  of  the 
matter,  I  have  not  yet  been  able  to  do  anything  with  it :  meanwhile,  I  put 
it  by.  On  looking  at  it  a  year  or  two  after,  I  was  dismayed  to  find  it  had 
lost  much  by  deliquescence.  Accordingly,  I  potted  it  up  in  salt  in  an 
earthen  dish,  with  iron  rim,  and  placed  it  on  an  upper  shelf  in  a  room  with 
an  Arnott  stove,  where  it  has  remained  ever  since. 

*  If  you  should  find  it  of  any  use  I  would  ask  you,  if  possible,  to  repeat 
my  experiment  as  described,  and  settle  that  point,  which  has  always  struck 
xne  as  a  very  important  one.7 


DEFECTS    OF   METHOD.  34:5 

ever  carbonic  acid  may  be  contained  in  the  arfc^  %'  is  a  sim- 
ilar tube,  filled  with  fragments  of  pumice  stone  moistened 
with  sulphuric  acid ;  it  is  intended  to  absorb  the  aqueous 
vapour  of  the  air.  Thus  the  air  reaches  the  cylinder  de- 
prived both  of  its  aqueous  vapour  and  its  carbonic  acid.  It 
is  now  entering, — the  mercury-gauge  of  the  pump  is  de- 
scending, and  as  it  enters  I  would  beg  of  you  to  observe 
the  needle.  If  the  entrance  of  the  air  diminish  the  radia- 
tion through  the  cylinder — -if  air  be  a  substance  which  is 
competent  to  destroy  the  waves  of  ether  in  any  sensible 
degree — this  will  be  declared  by  the  diminished  deflection 
of  the  galvanometer.  The  tube  is  now  full,  but  you  see  no 
change  in  the  position  of  the  needle,  nor  could  you  see  any 
change  even  if  you  were  close  to  the  instrument.  The  air 
thus  examined  seems  as  transparent  to  radiant  heat  as  the 
vacuum  itself. 

By  changing  the  screen  I  can  alter  the  amount  of  heat 
falling  upon  the  pile ;  thus,  by  withdrawing  it,  I  can  cause 
the  needle  to  stand  at  40°,  50°,  60°,  70°  and  80°  in  succes- 
sion ;  and  while  it  occupies  each  position  I  can  repeat  the 
experiment  which  I  have  just  performed  before  you.  In 
no  instance  could  you  recognize  the  slightest  movement  of 
the  needle.  The  same  is  the  case  if  I  push  the  screen  for- 
ward, so  as  to  reduce  the  deflection  to  20  and  10  degrees. 

The  experiment  just  made  is  a  question  addressed  to 
Nature,  and  her  silence  might  be  construed  into  a  negative 
reply.  But  a  natural  philosopher  must  not  lightly  accept  a 
negative,  and  I  am  not  sure  that  we  have  put  our  question 
in  the  best  possible  language.  Let  us  analyse  what  we 
have  done,  and  first  consider  the  case  of  our  smallest  de- 
flection of  10  degrees.  Supposing  that  the  air  is  not  per- 
fectly diathermic  ;  that  it  really  intercepts  a  small  portion 
— say  the  thousandth  part  of  the  heat  passing  through  the 
tube — that  out  of  every  thousand  rays  it  struck  down  one  ; 
should  we  be  able  to  detect  this  execution  ?  This  absorp- 


34:6  LECTURE   X. 

tion,  if  it  took  place,  would  lower  the  deflection  the  thou- 
sandth part  of  ten  degrees,  or  the  hundredth  part  of  one 
degree,  a  diminution  which  it  would  be  impossible  for  you 
to  see,  even  if  you  were  close  to  the  galvanometer.*  In 
the  case  here  supposed,  the  total  quantity  of  heat  falling 
upon  the  pile  is  so  inconsiderable,  that  a  small  fraction  of 
it,  even  if  absorbed,  might  well  escape  detection. 

But  we  have  not  confined  ourselves  to  a  small  quantity 
of  heat ;  the  result  was  the  same  when  the  deflection  was 
80°  as  when  it  was  10°.  Here  I  must  ask  you  to  sharpen 
your  attention  and  accompany  me,  for  a  time,  over  rather 
difficult  ground.  I  want  now  to  make  clearly  intelligible 
to  you  an  important  peculiarity  of  the  galvanometer. 

The  needle  being  at  zero,  let  us  suppose  a  quantity  of 
heat  to  fall  upon  the  pile,  sufficient  to  produce  a  deflection 
of  one  degree.  Suppose  that  I  afterwards  augment  the 
quantity  of  heat,  so  as  to  produce  deflections  of  two  de- 
grees, three  degrees,  four  degrees,  five  degrees ;  I  then 
know  that  the  quantities  of  heat  which  produce  these  de- 
flections stand  to  each  other  in  the  ratios  of  1  :  2  :  3  :  4  :  5 ; 
the  quantity  of  heat  which  produces  a  deflection  of  5°  be- 
ing exactly  five  times  that  which  produces  a  deflection  of 
1°.  But  this  proportionality  exists  only  so  long  as  the  de- 
flections do  not  exceed  a  certain  magnitude.  For,  as  the 
needle  is  drawn  more  and  more  aside  from  zero,  the  cur- 
rent acts  upon  it  at  an  ever  augmenting  disadvantage. 
The  case  is  illustrated  by  a  sailor  working  a  capstan ;  he 
always  applies  his  strength  at  right  angles  to  the  lever, 
for,  if  he  applied  it  obliquely,  only  a  portion  of  that  strength 
would  be  effective  in  turning  the  capstan  round.  And  in 
the  case  of  our  electric  current,  when  the  needle  is  very 
oblique  to  the  current's  direction,  only  a  portion  of  its  force 

*It  will  be  borne  in  mind  that  I  am  here  speaking  of  galvanometric 
not  of  thermomctric  degrees. 


RELATION   OF   DEFLECTION  TO   ABSORPTION.  34:7 

is  effective  in  moving  the  needle  round.  Thus  it  happens, 
that  though  the  quantity  of  heat  may  be,  and,  in  our  case, 
is,  accurately  expressed  by  the  strength  of  the  current 
which  it  excites,  still  the  larger  deflections,  inasmuch  as 
they  do  not  give  us  the  action  of  the  whole  current,  but 
only  of  a  part  of  it,  cannot  be  a  true  measure  of  the 
amount  of  heat  falling  upon  the  pile. 

The  galvanometer  now  before  you  is  so  constructed 
that  the  angles  of  deflection,  up  to  30°  or  thereabouts,  are 
proportional  to  the  quantities  of  heat ;  the  quantity  neces- 
sary to  move  the  needle  from  30°  to  31°  is  nearly  the  same 
as  that  required  to  move  it  from  0°  to  1°.  But  beyond  30° 
the  proportionality  ceases.  The  quantity  of  heat  required 
to  move  the  needle  from  40°  to  41°  is  three  times  that  ne- 
cessary to  move  it  from  0°  to  1°  ;  to  deflect  it  from  50°  to 
51°  requires  five  times  the  heat  necessary  to  move  it  from 
0°  to  1°  ;  to  deflect  it  from  60°  to  61°  requires  about  ten 
times  the  heat  necessary  to  move  it  from  0°  to  1°  ;  to  de- 
flect it  from  70°  to  71°  requires  nearly  twenty  times,  while 
to  move  it  from  80°  to  81°  requires  more  than  fifty  times 
the  heat  necessary  to  move  it  from  0°  to  1°.  Thus,  the 
higher  we  go,  the  greater  is  the  quantity  of  heat  represent- 
ed by  a  degree  of  deflection ;  the  reason  being,  that  the 
force  which  then  moves  the  needle  is  only  a  fraction  of  the 
force  really  circulating  in  the  wire,  and  hence  represents 
only  a  fraction  of  the  heat  falling  upon  the  pile. 

By  a  certain  process,  which  I  will  not  stop  here  to  de- 
scribe,* I  can  express  the  higher  degrees  in  terms  of  the 
lower  ones;  I  thus  learn,  that  while  deflections  of  10°,  20°, 
30°,  respectively,  express  quantities  of  heat  represented  by 
the  numbers  10,  20,  30,  a  deflection  of  40°  represents  a 
quantity  of  heat  expressed  by  the  number  47  ;  a  deflection 
of  50°  expresses  a  quantity  of  heat  expressed  by  the  num- 

*  See  Appendix  to  Lecture  X. 


34:8  LECTURE  X. 

ber  80 ;  while  the  deflections  60°,  Y0°,  80°,  express  quan- 
tities of  heat  which  increase  in  a  much  more  rapid  ratio 
than  the  deflections  themselves. 

What  is  the  upshot  of  this  analysis  ?  It  will  drive  us, 
I  think,  to  a  better  method  of  questioning  Nature.  It  leads 
to  the  reflection  that,  when  we  make  our  angles  small,  the 
quantity  of  heat  falling  on  the  pile  is  so  inconsiderable,  that 
even  if  a  fraction  of  it  were  absorbed,  it  might  escape  de- 
tection ;  while,  if  we  make  our  deflections  large,  by  em- 
ploying a  powerful  flux  of  heat,  the  needle  is  in  a  position 
from  which  it  would  require  a  considerable  addition  or 
abstraction  of  heat,  to  move  it.  The  1,000th  part  of  the 
whole  radiation  in  the  one  case  would  be  too  small,  abso- 
lutely, to  be  measured;  the  1,000th  part  in  the  other  case 
might  be  something  considerable,  without,  however,  being 
considerable  enough  to  affect  the  needle  in^any  sensible  de- 
gree. When,  for  example,  the  deflection  is  over  80°,  an 
augmentation  or  diminution  of  heat,  equivalent  to  15  or  20 
of  the  lower  degrees  of  the  galvanometer,  would  be  scarce- 
ly measurable. 

We  are  now  face  to  face  with  our  problem  ;  it  is  this, 
to  work  with  a  flux  of  heat  so  large  that  a  small  fractional 
part  of  it  will  not  be  infinitesimal,  and  still  to  keep  our 
needle  in  its  most  sensitive  position.  If  we  can  accom- 
plish this  we  shall  augment  indefinitely  our  experimental 
power.  If  a  fraction  of  the  heat,  however  small,  be  inter- 
cepted by  the  gas,  we  can  augment  the  absolute  value  of 
that  fraction  by  augmenting  the  total  of  which  it  is  a  frac- 
tion. 

The  problem,  happily,  admits  of  an  effective  practical 
solution.  You-  know  that  when  we  allow  heat  to  fall  upon 
the  opposite  faces  of  the  thermo-electric  pile,  the  currents 
generated  neutralise  each  other  more  or  less ;  and,  if  the 
quantities  of  heat  falling  upon  the  two  faces  be  perfectly 
equal,  the  neutralisation  is  complete.  Our  galvanometer 


IMPKOVED  APPARATUS.  349 

needle  is  now  deflected  to  80°  by  the  flux  of  heat  passing 
through  the  tube ;  I  uncover  the  second  face  of  the  pile, 
furnish  it  with  its  conical  reflector,  and  place  a  second  cube 
of  boiling  water  in  front  of  it ;  the  needle,  as  you  see,  de- 
scends instantly. 

By  means  of  a  proper  adjusting  screen  I  can  so  regulate 
the  quantity  of  heat  falling  upon  the  posterior  face  of  the 
pile,  that  it  shall  exactly  neutralise  the  heat  incident  upon 
its  other  face  :  this  is  now  effected ;  and  the  needle  points 
to  zero. 

Here,  then,  we  have  two  powerful  and  perfectly  equal 
fluxes  of  heat,  falling  upon  the  opposite  faces  of  the  pile, 
one  of  which  passes  through  our  exhausted  cylinder.  If  I 
allow  air  to  enter  the  cylinder,  and  if  this  air  exert  any  ap- 
preciable action  upon  the  rays  of  heat,  the  equality  now 
existing  will  be  destroyed ;  a  portion  of  the  rays  passing 
through  the  tube  being  struck  down  by  the  air,  the  second 
source  of  heat  will  triumph ;  the  needle,  now  in  its  most 
sensitive  position,  will  be  deflected ;  and  from  the  magni- 
tude of  the  deflection  we  can  accurately  calculate  the  ab- 
sorption. 

I  have  thus  sketched,  in  rough  outline,  the  apparatus  by 
which  our  researches  on  the  relation  of  radiant  heat  to 
gaseous  matter  must  be  conducted.  The  necessary  tests 
are,  however,  at  the  same  time  so  powerful  and  so  delicate, 
that  a  rough  apparatus  like  that  just  described  would  not 
answer  our  purpose.  But  you  will  now  experience  no  diffi- 
culty in  comprehending  the  construction  and  application  of 
the  more  perfect  apparatus,  with  which  the  experiments  on 
gaseous  absorption  and  radiation  have  been  actually  made. 
See  Plate  I.,  at  the  end  of  the  volume. 

Between  s  and  s'  stretches  the  experimental  cylinder,  a 
hollow  tube  of  brass,  polished  within ;  at  s,  and  s',  are  the 
plates  of  rock  salt  which  close  the  cylinder  air-tight ;  the 
length  from  s  to  s',  in  the  experiments  to  be  first  recorded, 


350  LECTURE    X. 

is  4  feet,  c,  the  source  of  heat,  is  a  cube  of  cast  copper, 
filled  with  water,  which  is  kept  continually  boiling  by  the 
lamp  L.  Attached  to  the  cube  c  by  brazing  is  the  short 
cylinder  r,  of  the  same  diameter  as  the  experimental  cylin- 
der, and  capable  of  being  connected,  air-tight  with  the  lat- 
ter at  s.  Thus  between  the  source  c  and  the  end  s'  of  the 
experimental  tube,  we  have  the  front  chamber  F,  from 
which  the  air  can  be  removed,  so  that  the  rays  from  the 
source  will  enter  the  cylinder  s  s'  unsifted.  To  prevent 
the  heat  from  the  source  c  passing  by  conduction  to  the 
plate  at  s,  the  chamber  F  is  caused  to  pass  through  the 
vessel  v,  in  which  a  stream  of  cold  water  continually  circu- 
lates, entering  through  the  pipe  i  i,  which  dips  to  the  bot- 
tom of  the  vessel,  and  escaping  through  the  waste-pipe  e  e. 
The  experimental  tube  and  the  front  chamber  are  connect- 
ed, independently,  with  the  air-pump  A  A,  so  that  either  of 
them  may  be  exhausted  or  filled  without  interfering  with 
the  other.  I  may  remark  that  in  later  arrangements  the 
experimental  cylinder  was  supported  apart  from  the  pump, 
being  connected  with  the  latter  by  a  flexible  tube.  The 
tremulous  motion  of  the  pump,  which  occurred  when  the 
connection  was  rigid,  was  thus  completely  avoided. 

p  is  the  thermo-electric  pile,  placed  on  its  stand  at  the 
end  of  the  experimental  tube,  and  furnished  with  its  two 
conical  reflectors,  c'  is  the  compensating  cube,  used  to 
neutralise  the  radiation  from  c ;  n  is  the  adjusting  screen, 
which  is  capable  of  an  exceedingly  fine  motion  to  and  fro. 
N  isr  is  a  delicate  galvanometer  connected  with  the  pile  P, 
by  the  wires  w  w'.  The  graduated  tube  o  o  (to  the  right 
of  the  plate),  and  the  appendage  M  K  (attached  to  the  cen- 
tre of  the  experimental  tube)  shall  be  referred  to  more  par- 
ticularly by  and  by. 

I  should  hardly  sustain  your  interest  in  stating  the  diffi- 
culties which  at  first  beset  the  investigation  conducted  with 
this  apparatus,  or  the  numberless  precautions  which  the 


ACTION   OF   ATMOSPHERIC   AIR.  351 

exact  balancing  of  the  two  powerful  sources  of  heat,  here 
resorted  to,  rendered  necessary.  I  believe  the  experiments 
made  with  atmospheric  air  alone  might  be  numbered  by 
tens  of  thousands.  Sometimes  for  a  week,  or  even  for  a 
fortnight,  coincident  and  satisfactory  results  would  be  ob- 
tained ;  the  strict  conditions  of  accurate  experimenting 
would  appear  to  be  found,  when  an  additional  day's  ex- 
perience would  destroy  the  superstructure  of  hope,  and  ne- 
cessitate a  recommencement,  under  changed  conditions,  of 
the  whole  enquiry.  It  is  this  which  daunts  the  experi- 
menter ;  it  is  this  preliminary  fight  with  the  entanglements 
of  a  subject,  so  dark,  so  doubtful,  so  uncheering ;  without 
any  knowledge  whether  the  conflict  is  to  lead  to  anything 
worth  possessing,  that  renders  discovery  difficult  and  rare. 
But  the  experimenter,  and  particularly  the  young  experi- 
menter, ought  to  know,  that  as  regards  his  own  moral  man- 
hood, he  cannot  but  win  if  he  only  contend  aright.  Even  with 
a  negative  result,  the  consciousness  that  he  has  gone  fair- 
ly to  the  bottom  of  his  subject,  as  far  as  his  means  allowed 
— the  feeling  that  he  has  not  shunned  labour,  though  that 
labour  may  have  resulted  in  laying  bare  the  nakedness  of 
his  case — reacts  upon  his  own  mind,  and  gives  it  firmness 
for  future  work. 

But  to  return ; — I  first  neglected  atmospheric  vapour 
and  carbonic  acid  altogether,  concluding,  as  others  did 
afterwards,  that  the  quantities  of  these  substances  being 
so  small,  their  effect  upon  radiant  heat  must  be  quite  in- 
appreciable ;  after  a  time,  however,  I  found  this  assump- 
tion leading  me  quite  astray.  I  first  used  chloride  of  cal- 
cium as  a  drying  agent,  but  had  to  abandon  it.  I  next  used 
pumice  stone  moistened  with  sulphuric  acid,  and  had  to 
give  it  up  also.  I  finally  resorted  to  pure  glass  broken  to 
small  fragments,  wetted  with  sulphuric  acid,  and  inserted 
by  means  of  a  funnel  into  a  U  tube.  I  found  this  arrange- 
ment best,  but  even  here  the  greatest  care  was  needed.  It 


352  LECTURE   X. 

was  necessary  to  cover  each  column  with  a  layer  of  dry 
glass  fragments,  for  I  found  that  the  smallest  particle  of 
dust  from  the  cork,  or  a  quantity  of  sealing  wax  not  more 
than  the  twentieth-part  of  a  pin's  head  in  size,  was  quite 
sufficient,  if  it  reached  the  acid,  to  vitiate  the  results.  The 
drying-tubes  moreover  had  to  be  frequently  changed,  as 
the  organic  matter  of  the  atmosphere,  infinitesimal  though 
it  was,  soon  introduced  disturbance. 

To  remove  the  carbonic  acid,  pure  Carrara  marble  was 
broken  into  fragments,  wetted  with  caustic  potash,  and  in- 
troduced into  a  U  tube.  These,  then,  are  the  agents  for 
drying  the  gas  and  removing  the  carbonic  acid  which  are 
used  at  present ;  but  previous  to  their  final  adoption,  I  em- 
ployed, to  dry  the  air,  the  arrangement  shown  in  Plate  I., 
where  the  glass  tubes  marked  Y  T,  each  three  feet  long, 
were  filled  with  chloride  of  calcium,  after  which  were 
placed  two  U  tubes  K  z,  filled  with  pumice  stone  and  sul- 
phuric acid.  Hence,  the  air,  in  the  first  place,  had  to  pass 
over  1 8  feet  of  chloride  of  calcium,  and  afterwards  through 
the  sulphuric  acid  tubes,  before  it  entered  the  experimental 
tube  s  s'.  A  gas-holder,  G  G,  was  employed  for  other 
gases  than  atmospheric  air.  In  the  investigation  on  which 
I  am  at  present  engaged,  this  arrangement,  as  I  have  said, 
is  abandoned,  a  simpler  one  being  found  more  effectual. 

My  assistant  has  now  exhausted  both  the  front  chamber 
F  and  the  experimental  tube  s  s'.  The  rays  are  passing 
from  the  source  c  through  the  front  chamber ;  across  the 
plate  of  rocksalt  at  s,  through  the  experimental  tube,  across 
the  plate  at  s',  afterwards  impinging  upon  the  anterior  sur- 
face of  the  pile  P.  This  radiation  is  neutralised  by  that 
from  the  compensating  cube  c'.  The  needle,  you  will  ob- 
serve, is  at  zero.  We  will  commence  our  experiments  by 
applying  this  powerful  test  to  dry  air.  It  is  now  entering 
the  experimental  cylinder ;  but,  at  your  distance,  you  see 
no  motion  of  the  needle,  and  thus  our  more  powerful  mode 


ABSOKPTTON   OF   KADIANT   HEAT  BY   OLEFIANT   GAS.      353 

of  experiment  fails  to  detect  any  absorption  on  the  part  of 
the  air.  Its  atoms,  apparently,  are  incompetent  to  shatter 
a  single  calorific  wave  ;  it  is  a  practical  vacuum,  as  regards 
the  rays  of  heat.  Were  you  quite  near,  however,  you 
would  see  a  deflection  of  the  needle  amounting  to  about 
one  degree.  Oxygen,  hydrogen,  and  nitrogen,  when  care- 
fully purified,  exhibit  the  action  of  atmospheric  air ;  they 
are  almost  neutral. 

But  the  neutral  quality  of  atmospheric  air  was  thought 
to  extend  to  transparent  gases  generally.  Let  us  see 
whether  this  is  correct.  I  have  here  a  gas-holder  of  olefiant 
gas, — common  coal  gas  would  also  answer  my  purpose.  I 
discharge  a  little  of  the  olefiant  gas  in  the  air,  but  you  see 
nothing;  the  gas  is  perfectly  transparent.  The  experi- 
mental tube  is  exhausted,  and  the  needle  points  to  zero ; 
and  now  we  will  allow  the  olefiant  gas  to  enter.  Observe 
the  effect.  The  needle  moves  in  a  moment ;  the  transpar- 
ent gas  strikes  down  the  rays  wholesale — the  final  and  per- 
manent deflection,  when  the  tube  is  full,  amounting  to  70 
degrees. 

I  will  now  interpose  a  metal  screen  between  the  pile  P 
and  the  end  s'  of  the  experimental  tube,  thus  entirely  cut- 
ting off  the  radiation  through  the  tube.  The  face  of  the 
pile  turned  towards  the  metal  screen  wastes  its  heat  speed- 
ily by  radiation ;  it  is  now  at  the  temperature  of  this 
room,  and  the  radiation  from  the  compensating  cube  alone 
acts  on  the  pile,  producing  a  deflection  of  75  degrees.  But 
at  the  commencement  of  the  experiment  the  radiations  from 
both  cubes  were  equal,  hence  the  deflection  75°  corresponds 
to  the  total  radiation  through  the  experimental  tube,  when 
the  latter  is  exhausted. 

Taking  as  unit  the  quantity  of  heat  necessary  to  move 
the  needle  from  0°  to  1°,  the  number  of  units  expressed  by 
a  deflection  of  75°  is 

276. 


354:  LECTURE   X. 

* 

The  number  of  units  expressed  by  a  deflection  of  70°  is 
211. 

Out  of  a  total,  therefore,  of  276,  olefiant  gas  has  struck 
down  211;  that  is  about  seven-ninths  of  the  whole,  or 
about  80  per  cent. 

Does  it  not  seem  to  you  as  if  an  opaque  layer  had  been 
suddenly  precipitated  on  our  plates  of  salt,  when  the  gas 
entered  ?  The  substance,  however,  deposits  no  such  layer. 
I  discharge  a  current  of  the  dried  gas  against  a  polished 
plate  of  salt,  but  you  do  not  perceive  the  slightest  dimness. 
The  rocksalt  plates,  moreover,  though  necessary  for  exact 
measurements,  are  not  necessary  to  show  the  destructive 
powers  of  this  gas.  Here  is  an  open  tin  cylinder  which  I 
interpose  between  the  pile  and  our  radiating  source ;  I 
force  olefiant  gas  gently  into  the  cylinder  from  this  gas- 
holder and  you  see  the  needle  fly  up  to  its  stops.  Observe 
the  smallness  of  the  quantity  of  gas  which  I  shall  next  use. 
I  cleanse  the  open  tube  by  forcing  a  current  of  air  through 
it ;  the  needle  is  now  at  zero  ;  and  I  will  simply  turn  this 
cock  on  and  off,  as  speedily  as  I  can.  A  mere  bubble  of 
the  gas  enters  the  tube  in  this  brief  interval ;  still  you  see 
that  its  presence  causes  the  needle  to  swing  to  70°.  I  next 
abolish  the  open  tube,  and  leave  nothing  but  the  free  air 
between  the  pile  and  source ;  from  the  gasometer  I  dis- 
charge olefiant  gas  into  this  space.  You  see  nothing  in  the 
air,  but  the  swing  of  the  needle  through  an  arc  of  60°  de- 
clares the  presence  of  this  invisible  barrier  to  the  calorific 
rays. 

Thus,  it  is  shown  that  the  ethereal  undulations  which 
glide  among  the  atoms  of  oxygen,  nitrogen,  and  hydrogen, 
without  hindrance,  are  powerfully  absorbed  by  the  mole- 
cules of  olefiant  gas.  We  shall  find  other  transparent  gases 
also  almost  immeasurably  superior  to  air.  We  can  limit  at 
pleasure  the  number  of  the  gaseous  atoms,  and  thus  vary 
the  amount  of  destruction  of  the  ethereal  waves.  In  this 


RELATION   OF   QUANTITY   TO   ABSORPTION.  355 

respect  gaseous  bodies  possess  a  great  advantage  over 
liquids  and  solids,  in  experiments  on  radiation.  Attached 
to  the  air-pump  is  a  barometric  tube,  by  means  of  which  I 
can  admit  measured  portions  of  the  gas.  The  experimen- 
tal cylinder  is  now  exhausted,  and  turning  this  cock  slowly 
on,  and  observing  the  mercury  gauge,  I  allow  the  olefiant 
gas  to  enter,  till  the  mercurial  column  has  been  depressed 
an  inch.  I  observe  the  galvanometer  and  read  the  deflec- 
tion. Determining  thus  the  absorption  produced  by  one 
inch,  another  inch  is  added,  and  the  absorption  effected  by 
two  inches  of  the  gas  is  determined.  Proceeding  thus  we 
obtain  for  tensions  from  1  to  10  inches  the  following  ab- 
sorptions : — 

Olefiant  Gas. 

Tensions 

in  inches  Absorption 

1  .  .  .  .90 

2  .  .  .  .123 

3  .  .  .  .142 

4  .  .  .  .157 

6  .  .  .  .168 

6        .        .        .        .   m 

7  .  .  .  .182 

8  .  .  .  .186 

9  .  .  .  .190 
10  .  .  .  .193 

The  unit  here  used  is  the  amount  of  heat  absorbed 
when  a  whole  atmosphere  of  dried  air  is  allowed  to  enter 
the  tube.  The  table,  for  example,  shows  that  one-thirtieth 
of  an  atmosphere  of  olefiant  gas  exercises  ninety  times  the 
absorption  of  a  whole  atmosphere  of  air. 

The  table  also  informs  us  that  each  additional  inch  of 
olefiant  gas  produces  less  destruction  than  the  preceding 
one.  A  single  inch,  at  the  commencement,  strikes  down 
90  rays,  but  a  second  inch  strikes  down  only  33,  while  the 
addition  of  an  inch,  when  nine  inches  are  already  in  the 


356  LECTURE   X. 

tube,  effects  the  destruction  of  only  3  rays.  This  is  what 
might  reasonably  be  expected.  The  number  of  rays  emit- 
ted is  finite,  and  the  discharge  of  the  first  inch  of  olefiant 
gas  amongst  them  has  so  thinned  their  ranks  that  the  exe- 
cution produced  by  the  second  inch  is  naturally  less  than 
that  of  the  first.  This  execution  must  diminish,  as  the 
number  of  rays  capable  of  being  destroyed  by  the  gas,  be- 
comes less ;  until,  finally,  all  absorbable  rays  being  removed, 
the  residual  heat  would  pass  through  the  gas  unimpeded.* 

But  supposing  the  quantity  of  gas  first  introduced  to  be 
so  inconsiderable,  that  the  number  of  rays  extinguished  by 
it  is  a  vanishing  quantity,  compared  with  the  total  number 
capable  of  being  destroyed,  we  might  then  reasonably  ex-- 
pect  that,  for  some  time  at  least,  the  quantity  of  execution 
done  would  be  proportional  to  the  quantity  of  gas  present. 
That  a  double  quantity  of  gas  would  produce  a  double 
effect,  a  treble  quantity  a  treble  effect ;  or,  in  general 
terms,  that  the  absorption  wrould,  for  a  time,  be  found  pro 
portional  to  the  density. 

To  test  this  idea  we  will  make  use  of  a  portion  of  the 
apparatus  omitted  in  the  general  description,  o  o  (Plate 
I.)  is  a  graduated  glass  tube,  the  end  of  which  dips  into  the 
basin  of  water  B.  The  tube  is  closed  above  by  means  of 
the  stopcock  r ;  d  d  is  a  tube  containing  fragments  of  chlo- 
ride of  calcium.  The  tube  o  o  is  first  filled  with  water  up 
to  the  cock  r,  and  the  water  is  afterwards  carefully  dis- 
placed by  olefiant  gas  admitted  in  bubbles  from  below. 
The  gas  is  admitted  into  the  experimental  cylinder  by  the 
cock  r,  and  as  it  enters,  the  water  rises  in  o  o,  each  of 
whose  divisions  represents  a  volume  of  3Vtn  of  a  cubic  inch. 
Successive  measures  of  this  capacity  are  permitted  to  enter 
the  tube,  and  the  absorption  in  each  particular  case  is  de- 
termined. 

In  the  following  table  the  first  column  contains  the 
quantity  of  gas  admitted  into  the  tube ;  the  second  con- 
*  See  Note  (7)  at  the  end  of  this  Lecture. 


ABSORPTION  BY   ETHEK  VAPOUK.  357 

tains  the  corresponding  absorption  ;  the  third  column  con- 
tains the  absorption,  calculated  on  the  supposition  that  it  is 
proportional  to  the  density. 

Olefiant  Gas. 

Unit  measure  j^tli  of  a  cubic  inch. 

Absorption. 

Measures  of  Gas. 
1 
2 
3 
4 

6  . 
6 

7  ... 
8 

9 

10 
11 
12 
13 
14 
15 

This  table  proves  the  correctness  of  the  surmise,  that 
when  very  small  quantities  of  the  gas  are  employed,  the 
absorption  is  sensibly  proportional  to  the  density.  But 
consider  for  a  moment  the  tenuity  of  the  gas  with  which 
we  have  here  operated.  The  volume  of  our  experimental 
tube  is  220  cubic  inches  ;  imagine  ^th  of  a  cubic  inch  of 
gas  diffused  in  this  space,  and  you  have  the  atmosphere 
through  which  the  calorific  rays  passed  in  our  first  experi- 
ment. This  atmosphere  possesses  a  tension  not  exceeding 
T \l o  otn  °f tnat  °f  ordinary  air.  It  would  depress  the  mer- 
curial column  connected  with  the  air-pump  not  more  than 
a£Tth  of  an  English  inch.  Its  action,  however,  upon  the 
calorific  rays  is  perfectly  measurable. 

But  the  absorptive  energy  of  olefiant  gas,  extraordinary 


Observed. 

Calculated. 

2-2 

2'2 

4-5 

4-4 

6-6 

6-6 

8-8 

8'8 

.     11-0 

.       ll'O 

.       12-0 

13-2 

14-8 

.       15.4 

16-8 

.       17'6 

19-8 

.       19-8 

.       22-0 

.       22-0 

.       24-0 

.       24-2 

.       25-4 

.       26-4 

.       29-0 

.       28-6 

.       30-2 

.       29-8 

33-5 

33-0 

358 


LECTURE   X. 


Fisr.  00. 


as  it  is  shown  to  be  by  the  foregoing  experiments,  is  ex- 
ceeded by  that  of  various  vapours,  the  action  of  which  I 
will  now  endeavour  to  illustrate.  Here  is  a  glass  flask,  G 
(fig.  90),  provided  with  a  brass  cap,  into  which  a  stopcock 
can  be  screwed  air-tight.  I  pour  a  small  quantity  of  sul- 
phuric ether  into  the  flask,  and  completely  re- 
move, in  the  first  place,  the  air  which  fills  the 
flask  above  the  liquid.  I  attach  the  flask  to 
the  experimental  tube,  which  is  now  exhaust- 
ed— the  needle  pointing  to  zero — and  permit 
the  vapour  from  the  flask  to  enter  the  experi- 
mental tube.  The  mercury  of  the  gauge 
sinks,  and  now  that  it  is  depressed  one  inch  I 
will  stop  the  further  supply  of  vapour.  The 
moment  the  vapour  entered,  the  needle  moved, 
and  it  now  points  to  65°.  I  can  add  another 
inch,  and  again  determine  the  absorption,  a 
third  inch  and  do  the  same.  The  absorptions 
effected  by  four  inches,  introduced  in  this  way,  are  given  in 
the  following  table.  For  the  sake  of  comparison  I  place 
the  corresponding  absorptions  of  olefiant  gas  in  the  third 
column. 

Sulphuric  Ether. 


Tensions 
in  inches. 

1 

2 
3 
4 


Absorption. 
.     214 
.     282 
.     315 

330 


Correspon diner  absorption 
of  Olefiant  Gas. 

90 
123 
142 
154 


For  these  tensions  the  absorption  of  radiant  heat  by  the 
vapour  of  sulphuric  ether  is  about  two  and  two-third  times 
the  absorption  of  olefiant  gas.  There  is,  moreover,  no  pro- 
portionality between  the  quantity  of  vapour  and  the  ab- 
sorption. 

But  reflections  similar  to  those  which  we  have  already 
applied  to  olefiant  gas  are  also  applicable  to  the  ether. 
Supposing  we  make  our  unit  measure  small  enough,  the 


RELATION   OF   QUANTITY  TO   ABSORPTION.  Oi>9 

number  of  rays  first  destroyed  will  vanish  in  comparison 
with  the  total  number,  and,  for  a  time,  the  fact  will  proba- 
bly manifest  itself,  that  the  absorption  is  directly  propor- 
tional to  the  density.  To  examine  whether  this  is  the  case, 
the  other  portion  of  the  apparatus,  omitted  in  the  general 
description,  was  made  use  of.  K  is  one  of  the  small  flasks 
already  described,  with  a  brass  cap,  which  is  closely 
screwed  on  to  the  stopcock  c'.  Between  the  cocks  c'  and 
c,  which  latter  is  connected  with  the  experimental  tube,  is 
the  chamber  M,  the  capacity  of  which  was  accurately  deter- 
mined. The  flask  Jc  was  partially  filled  with  ether,  and  the 
air  above  the  liquid  removed.  The  stopcock  c'  being  shut 
oif  and  c  turned  on,  the  tube  s  s'  and  the  chamber  M  are 
exhausted.  The  cock  c  is  now  shut  off,  and  c'  being  turned 
on,  the  chamber  M  becomes  filled  with  pure  ether  vapour. 
By  turning  c'  off  and  c  on,  this  quantity  of  vapour  is  allow- 
ed to  diffuse  itself  through  the  experimental  tube,  where 
its  absorption  is  determined  ;  successive  measures  are  thus 
sent  into  the  tube,  and  the  effect  produced  by  each  is  noted. 
In  the  following  table  the  unit  measure  made  use  of  had 
a  volume  of  T£  ^th  of  a  cubic  inch. 

SulpJmric  Ether. 

Absorption. 

Measures. 
1 

2  ... 

4  ... 

5  ... 

6  ... 

7  ... 

8  ... 

9  ... 

10  ... 

11  ... 

12  ... 

13  ... 

14  ... 
15 


Observed. 
5-0         ... 

Calculated. 
4-6 

.       10-3 

9-2 

.       19-2 

.       18-4 

24-5 

23-0 

29-5 

27-0 

34-5 

.       32-2 

38-0 

36-8 

.       44-0         .         ... 

.       41-4 

,       46-2 

.       46-2 

.       60-0 

60-6 

52-8 

55-2 

55-0 

.       59-8 

.       57-2 

.       64-4 

59-4 

69-0 

360  LECTURE  X. 

We  here  find  that  the  proportion  between  density  and 
absorption  holds  sensibly  good  for  the  first  eleven  measures, 
after  which  the  deviation  from  proportionality  gradually 


augments. 


No  doubt,  for  smaller  measures  than'-fi^th  of  a  cubic 
inch  the  above  law  holds  still  more  rigidly  true ;  and  in 
a  suitable  locality  it  would  be  easy  to  determine,  with  per- 
fect accuracy,  y^th  of  the  absorption  produced  by  the  first 
measure  ;  this  would  correspond  to  yoV^h  of  a  cubic  inch 
of  vapour.  But,  before  entering  the  tube,  the  vapour  had 
only  the  tension  due  to  the  temperature  of  the  laboratory, 
namely  12  inches.  This  would  require  to  be  multiplied  by 
2 '5  to  bring  it  up  to  that  of  the  atmosphere.  Hence  the 
ToV  o^h  of  a  cubic  inch  would,  on  being  diffused  through  a 
tube  possessing  a  capacity  of  220  cubic  inches,  have  a  ten- 
sion of  li-oXa-flXiVoo  —  sooWT»tn  of  an  atmosphere  ! 

These  experiments  with  ether  and  olefiant  gas  show  that 
not  only  do  gaseous  bodies,  at  the  ordinary  tension  of  the 
atmosphere,  offer  an  impediment  to  the  transmission  of  ra- 
diant heat ;  not  only  are  the  interstitial  spaces  of  such  gases 
incompetent  to  allow  the  ethereal  undulations  free  passage ; 
but,  also,  that  their  density  may  be  reduced  vastly  below 
that  which  corresponds  to  the  atmospheric  pressure,  and 
still  the  cloor  thus  opened  is  not  wide  enough  to  let  the  un- 
dulations through.  There  is  something  in  the  constitution 
of  the  individual  molecules,  thus  sparsely  scattered,  which 
enables  them  to  destroy  the  calorific  waves.  The  destruc- 
tion, however,  is  merely  one  of  form  ;  there  is  no  absolute 
loss.  Through  dry  air  the  heat  rays  pass  without  sensibly 
warming  it;  through  olefiant  gas  and  ether  vapour  they 
cannot  pass  thus  freely ;  but  every  wave  withdrawn  from 
the  radiant  sheaf  produces  its  equivalent  motion  in  the  body 
of  the  absorbing  gas,  and  raises  its  temperature.  It  is  a 
case  of  transference,  not  of  annihilation.  I  might  extend 
the  experiments  to  all  available  volatile  liquids,  and  show 


ABSORPTION  BY  GASES.  361 

you  that  the  same  rule  holds  good  for  the  vapours  of 
all. 

Before  changing  the  source  of  heat  here  made  use  of,  I 
wish  to  direct  your  attention  for  a  moment  to  the  action 
of  a  few  of  the  permanent  gases  on  radiant  heat.  To 
measure  the  quantities  introduced  into  the  experimental 
tube,  the  mercury  gauge  of  the  air-pump  was  made  use  of. 
In  the  case  of  carbonic  oxide,  the  following  absorptions 
correspond  to  the  tensions  annexed  to  them,  the  action  of 
a  full  atmosphere  of  air,  which,  as  you  remember,  produces 
a  deflection  of  1°,  being  taken  as  unit : — 
Carbonic  Oxide. 

Absorption. 
Tension 
in  inches. 

0.5  ... 

1-0  ... 

1-5 

2-0  ... 

2'5  ... 

3-0  ... 

3-5  ... 

As  in  former  cases,  the  third  column  is  calculated  on  the 
assumption  that  the  absorption  is  directly  proportional  to 
the  density  of  the  gas ;  and  we  see  that  for  seven  measures, 
or  up  to  a  tension  of  3*5  inches,  the  proportionality  holds 
strictly  good.  But  for  large  quantities  this  is  not  the  case  ; 
when,  for  instance,  the  unit  measure  is  5  inches,  instead  of 
half-an-inch,  we  obtain  the  following  results : 

Absorption, 

Tension  , * » 

in  inches.  Observed.  Calculated. 

5         ....         18     ....         18 
10         .         .      .  .         .         32-5  ...         36 

15         .         .         .         .         45     .         .  .       .         •         54 

The  case  of  carbonic  oxide  is  therefore  similar  to  that  of 
olefiant  gas.     Carbonic  acid,  sulphide  of  hydrogen,  nitrous 
oxide,  and  other  gases,  though  differing  in  the  energy  of 
16 


Observed. 

Calculated. 

2-5 

2-5 

5-6 

5-0 

8-0 

7-5 

10-0 

.       10-0 

12-0 

.       12-5 

15-0 

.       15-0 

175 

17-5 

362  LECTURE   X. 

their  absorption,  and  all  of  them  exceeding  carbonic  oxide, 
exhibit,  when  small  and  large  quantities  are  used,  a  similar 
deportment  towards  radiant  heat. 

Thus,  then,  in  the  case  of  some  gases,  we  find  an  almost 
absolute  incompetence  on  the  part  of  their  atoms  to  be 
shaken  by  the  ethereal  waves.  They  remain  practically  at 
rest  when  the  undulations  speed  amongst  them,  while  the 
atoms  of  other  gases,  struck  by  these  same  undulations,  ab- 
sorb their  motion,  and  become  themselves  centres  of  heat. 
We  have  now  to  examine  what  gaseous  bodies  are  compe- 
tent to  do  in  this  latter  capacity ;  we  have  to  enquire 
whether  these  atoms  and  molecules,  which  can  accept  mo- 
tion from  the  ether  in  such  very  different  degrees,  are  not 
also  characterised  by  their  competency  to  impart  motion 
to  the  ether  in  different  degrees ;  or,  to  use  the  common 
language,  having  learned  something  of  the  power  of  differ- 
ent  gases,  as  absorbers  of  radiant  heat,  we  have  now  to  en- 
quire into  their  capacities  as  radiators. 

I  have  here  an  arrangement,  by  means  of  which  we  can 
put  the  necessary  question,  which  has  hitherto  received 
only  a  negative  reply.  P  (fig.  91)  is  the  thermo-electric  pile 
with  its  two  conical  reflectors  ;  s  is  a  double  screen  of  pol- 
ished tin ;  A  is  an  argand  burner,  consisting  of  two  concen- 
tric perforated  rings ;  c  is  a  copper  ball,  which,  during  the 
experiments,  is  heated  under  redness ;  while  the  tube  t  t 
leads  to  a  gas  holder.  When  the  hot  ball  c  is  placed  on 
the  burner  it  warms  the  air  in  contact  with  it ;  an  ascend- 
ing current  is  thus  established,  which,  to  some  extent,  acts 
upon  the  pile.  To  neutralise  this  action  a  large  Leslie's 
cube,  L,  filled  with  water,  a  few  degrees  above  the  air  in 
temperature,  is  placed  before  the  opposite  face  of  the  pile. 
The  needle  being  thus  brought  to  zero,  the  gas  is  forced, 
by  a  gentle  water  pressure,  through  the  orifices  of  the 
burner ;  it  meets  the  ball  c,  glides  along  its  surface,  and 
ascends,  in  a  warm  current,  in  front  of  the  pile.  The  rays 


BADIATION  BY   GASES. 


363 


from  the  heated  gas  gush  forth  in  the  direction  of  the  ar- 
rows against  the  pile,  and  the  consequent  deflection  of  the 

Fig.  91. 


galvanometer  needle  indicates  the  magnitude  of  the  radia- 
tion. 

The  results  of  the  experiments  are  given  in  the  second 
column  of  the  following  table,  the  numbers  there  recorded 
marking  the  extreme  limit  to  which  the  needle  swung, 
when  the  rays  from  the  gas  fell  upon  the  pile  : — 

Kadiation.  Absorption. 

0  0-2 


Air       . 
Oxygen 
Nitrogen 
Hydrogen 
Carbonic  oxide 
Carbonic  acid  . 
Nitrous  oxide  . 
Olefiant  gas 


0 

0 

0 

12 

18 

29 

53 


0-2 

0-2 

0-2 

18-0 

25-0 

44-0 

610 


364 


LECTUKE   X. 


In  order  to  compare  the  radiation  with  the  absorption, 
I  have  placed  in  the  third  column  the  deflections  due  to  the 
absorption  of  the  same  gases,  at  a  common  tension  of  5 
inches.  We  see  that  radiation  and  absorption  go  hand  in 
hand ;  that  the  molecule  which  shows  itself  competent  to 
intercept  a  calorific  flux,  shows  itself  competent,  in  a  pro- 
portionate degree,  to  generate  a  calorific  flux.  That,  in 
short,  a  capacity  to  accept  motion  from  tne  ether,  and  to 
impart  motion  to  the  ether,  by  gaseous  bodies,  are  correla- 
tive properties. 

And  here,  be  it  remarked,  we  are  relieved  from  all  con- 
siderations regarding  the  influence  of  cohesion,  on  the  re- 
sults. In  solids  and  liquids  the  particles  are  more  or  less 
in  thrall,  and  cannot  be  considered  as  individually  free. 

Fig.  92. 


The  difference  in  point  of  radiative  and  absorptive  power, 
between  alum  and  rocksalt,  for  example,  might  be  fairly 


BLACKNESS   OF   TRANSPARENT   GASES   TO   HEAT.       365 

regarded  as  due  to  their  character  as  aggregates,  held  to- 
gether by  crystallising  force.  But  the  difference  "between 
olefiant  gas  and  atmospheric  air  cannot  be  explained  in  this 
way ;  it  is  a  difference  dependent  on  the  individual  mole- 
cules of  these  substances,  and  thus  our  experiments  with 
gases  and  vapours  probe  the  question  of  atomic  constitution 
to  a  depth,  quite  unattainable  with  solids  and  liquids. 

I  have  refrained  thus  far  from  giving  you  as  full  a 
tabular  statement  of  the  absorptive  powers  of  gases  and 
vapours  as  the  experiments  made  with  the  apparatus  al- 
ready described  would  enable  me  to  do,  knowing  that  I 
had  in  reserve  results,  obtained  with  another  apparatus, 
which  would  better  illustrate  the  subject.  This  second  ar- 
rangement is  the  same  in  principle  as  the  first ;  only  two 
changes  of  importance  have  been  made  in  it.  The  first  is, 
that  instead  of  making  a  cube  of  boiling  water  my  source 
of  heat,  I  employ  a  plate  of  copper,  against  which  a  thin 
steady  gas-flame  from  a  Bunsen's  burner  is  caused  to  play ; 
the  heated  plate  forms  the  back  of  my  new  front  chamber, 
which  latter  can  be  exhausted  independently,  as  before. 
This  portion  of  the  apparatus  is  sketched  in  fig.  92,  the 
chimney  G  being  added.  The  second  alteration  is  the  sub- 
stitution of  a  tube  of  glass  of  the  same  diameter,  and  2 
feet  8  inches  long,  for  the  tube  of  brass  s  s',  Plate  I. 
All  the  other  parts  of  the  apparatus  remain  as  before.  The 
gases  were  introduced  in  the  manner  already  described  into 
the  experimental  tube,  and  from  the  galvanometric  deflec- 
tion, consequent  on  the  entrance  of  each  gas,  its  absorption 
was  calculated. 

The  following  table  gives  the  relative  absorptions  of 
several  gases,  at  a  common  tension  of  one  atmosphere : — 

Absorption  at 

Name.  30  inches  tension. 

Air         .  .  .  .1 

Oxygen  ....         1 


366  LECTURE   X. 

Absorption  of 
Name.  30  inches  tension. 

Nitrogen        ....  1 

Hydrogen       ....  1 

Chlorine        ....  39 

Hydrochloric  acid      ...  62 

Carbonic  oxide          ...  90 

Carbonic  acid  ...  90 

Nitrous  oxide  .  .  .         355 

Sulphide  of  hydrogen  .  .390 

Marsh  gas      .  .  403 

Sulphurous  acid         .  .  .         710 

Olefiantgas  .  .  .970 

Ammonia      .  .  .  .1195 

The  most  powerful  and  delicate  tests  that  I  have  been 
able  to  apply  have  not  yet  enabled  me  to  establish  a  differ- 
ence between  oxygen,  nitrogen,  hydrogen,  and  air.  The 
absorption  of  these  substances  is  exceedingly  small — prob- 
ably even  smaller  than  I  have  made  it.  The  more  perfect- 
ly the  above-named  gases  are  purified,  the  more  closely 
does  their  action  approach  to  that  of  a  vacuum.  And  who 
can  say  that  the  best  drying  apparatus  is  perfect  ?  I  can- 
not even  say  that  sulphuric  acid,  however  pure,  may  not 
yield  a  modicum  of  vapour  to  the  gases  passing  through  it, 
and  thus  make  the  absorption  by  those  gases  appear  great- 
er than  it  ought.  Stopcocks  also  must  be  greased,  and 
hence  may  contribute  an  infinitesimal  impurity  to  the  air 
passing  through  them.  But  however  this  may  be,  it  is 
certain  that  if  any  further  advance  should  be  made  in  the 
purification  of  the  more  feebly  acting  gases,  it  will  only 
serve  to  augment  the  enormous  differences  of  absorption 
exhibited  by  the  foregoing  table. 

Ammonia,  at  the  tension  of  an  atmosphere,  exerts  an 
absorption  at  least  1,195  times  that  of  the  air.  If  I  inter- 
pose this  metal  screen  between  the  pile  and  the  experiment- 
al tube,  the  needle  will  move  a  little,  but  so  little  that  you 
entirely  fail  to  see  it.  AYhat  does  this  experiment  mean  ? 


INFLUENCE   OF   ATOMIC   CONSTITUTION.  367 

Why,  that  this  ammonia  which,  within  our  glass  tube,  is 
as  transparent  to  light  as  the  air  we  breathe,  is  so  opaque 
to  the  heat  radiating  from  our  source,  that  the  addition  of 
a  plate  of  metal  hardly  augments  the  opacity.  I  have  rea- 
son to  believe  that  it  does  not  augment  it  at  all,  and  that 
this  light  transparent  gas  is  really  as  black,  at  the  present 
moment,  to  the  calorific  rays,  as  if  the  experimental  tube 
were  filled  with  ink,  pitch,  or  any  other  impervious  sub- 
stance. 

In  the  case  of  oxygen,  nitrogen,  hydrogen,  and  air,  the 
action  of  a  whole  atmosphere  is  so  small  that  it  would  be 
quite  useless  to  attempt  to  determine  the  action  of  a  fra& 
tional  part  of  an  atmosphere.  Could  we,  however,  make 
such  a  determination,  the  difference  between  them  and  the 
other  gases  would  come  out  still  more  forcibly  than  in  the 
last  table.  In  the  case  of  the  energetic  gases,  we  know 
that  the  calorific  rays  are  most  copiously  absorbed  by  the 
portion  of  gas  which  first  enters  the  experimental  tube,  the 
quantities  which  enter  last  producing,  in  many  cases,  a 
merely  infinitesimal  effect.  If,  therefore,  instead  of  com- 
paring the  gases  at  a  common  tension  of  one  atmosphere, 
we  were  to  compare  them  at  a  common  tension  of  an  inch, 
we  should  doubtless  find  the  difference  between  the  least 
absorbent  and  the  most  absorbent  gases  greatly  augment- 
ed. We  have  already  learned  that  for  small  quantities,  the 
heat  absorbed  is  proportional  to  the  amount  of  gas  present. 
Assuming  this  to  be  true  for  air  and  the  other  feeble  gases 
referred  to ;  taking,  that  is,  their  absorption  at  1  inch  of 
tension  to  be  Jgth  of  that  at  30  inches,  we  have  the  follow- 
ing comparative  effects.  It  will  be  understood  that  in  every 
case,  except  the  first  four,  the  absorption  of  1  inch  of  the 
gas  was  determined  by  direct  experiment. 

Absorption  nt 
Name.  1  inch  tension. 

Air  .  .  .  .1 

Oxygen.  ....         1 


368  LECTURE  X. 

Absorption  of 

Name.  2  inch  tension. 

Nitrogen  ....        1 

Hydrogen          .  .  .  .1 

Chlorine  .  .  .  .60 

Bromine  .  .  .  .160 

Carbonic  oxide  .  .  .    750 

Hydrobromic  acid          ,  .  .  1005 

Nitric  oxide      .  .  .  .1590 

Nitrous  oxide    ....  1860 
Sulphide  of  hydrogen    .  .  .  2100 

Ammonia          .  .  .  .  7260 

Olefiantgas       .  .  .  7950 

Sulphurous  acid  .  .  .  8800 

What  extraordinary  differences  in  the  constitution  and 
character  of  the  ultimate  particles  of  various  gases  do  the 
above  results  reveal!  For  every  individual  ray  struck 
down  by  the  air,  oxygen,  hydrogen,  or  nitrogen — the  am- 
monia strikes  down  a  brigade  of  7,260  rays ;  the  olefiant 
gas  a  brigade  of  7,950  ;  while  the  sulphurous  acid  destroys 
8,800.  "With  these  results  before  us,  we  can  hardly  help 
attempting  to  visualise  the  atoms  themselves,  trying  to  dis- 
cern, with  the  eye  of  intellect,  the  actual  physical  qualities 
on  which  these  vast  differences  depend.  These  atoms  are 
particles  of  matter,  plunged  in  an  elastic  medium,  accepting 
its  motions  and  imparting  their  motions  to  it.  Is  the  hope 
unwarranted,  that  we  may  be  able  finally  to  make  radiant 
heat  such  a  feeler  of  atomic  constitution,  that  we  shall  bo 
able  to  infer  from  their  action  upon  it,  the  mechanism  of 
the  ultimate  particles  of  matter  themselves  ? 

Have  we  even  now  no  glimpse  of  the  atomic  qualities 
necessary  to  form  a  good  absorber  ?  You  remember  our 
experiments  with  gold,  silver,  and  copper ;  you  recollect 
how  feebly  they  radiate  and  how  feebly  they  absorb.  We 
heated  them  by  boiling  water ;  that  is  to  say,  we  imparted, 
by  the  contact  of  the  water,  motion  to  their  atoms  ;  still 
this  motion  was  imparted  with  extreme  slowness  to  the 


INFLUENCE   OF   ATOMIC   CONSTITUTION.  369 

ether  in  which  those  atoms  swung.  That  their  particles 
glide  through  the  ether  with  scarcely  any  resistance  may 
also  be  inferred  from  the  length  of  tune  which  they  require 
to  cool  in  vacuo.  But  we  have  seen  that  when  the  motion 
which,  the  atoms  of  the  above  bodies  possess,  and  which 
they  are  incompetent  to  transfer  to  the  ether,  is  imparted, 
by  contact,  to  a  coat  of  varnish,  or  to  a  coat  of  chalk  or 
lampblack,  or  even  to  flannel  or  velvet,  these  bodies  soon 
waste  the  motion  on  the  ether.  The  same  we  found  true 
for  glass  and  earthenware. 

In  what  respect  do  those  good  radiators  difler  from  the 
metals  referred  to  ?  In  one  profound  particular — the  met- 
als are  elements  ;  the  others  are  compounds.  In  the  metals 
the  atoms  swung  singly ;  in  the  varnish,  velvet,  earthen- 
ware and  glass,  they  swung  in  groups.  And  now,  in  bodies 
as  diverse  from  the  metals  as  can  possibly  be  conceived,  we 
find  the  same  significant  fact  making  its  appearance.  Oxy- 
gen, hydrogen,  nitrogen,  and  air,  are  elements,  or  mixtures 
of  elements,  and,  both  as  regards  radiation  and  absorption, 
their  feebleness  is  declared.  They  swing  in  the  ether  with 
scarcely  any  loss  of  moving  force.  They  bear  the  same 
relation  to  the  compound  gases  as  a  smooth  cylinder- turn- 
ing in  water  does  to  a  paddle-wheel.  They  create  a  small 
comparative  disturbance. 

We  may  push  these  considerations  still  further.  It  is 
impossible  not  to  be  struck  by  the  position  of  chlorine  and 
bromine  in  the  last  table.  Chlorine  is  an  extremely  dense 
and  coloured  gas ;  bromine  is  a  far  more  densely-coloured 
vapour ;  still  we  find  them,  as  regards  perviousness  to  the 
heat  of  our  source,  standing  above  every  transparent  com- 
pound gas  in  the  table.  The  act  of  combination  with  hy- 
drogen produces,  in  the  case  of  each  of  these  substances,  a 
transparent  compound ;  but  the  chemical  act,  which  aug- 
ments the  transparency  to  light,  augments  the  opacity  to 
1C* 


370  LECTURE   X. 

heat ;  hydrochloric  acid  absorbs  more  than  chlorine ;  and 
hydrobromic  acid  absorbs  more  than  bromine. 

Further,  I  have  here  the  element  bromine  in  the  liquid 
condition ;  I  enclose  a  portion  of  it  in  this  glass  cell ;  the 
layer  is  of  a  thickness  sufficient  to  extinguish  utterly  the 
flame  of  a  lamp  or  candle.  But  I  place  a  candle  in  front 
of  the  cell,  and  a  thermo-electric  pile  behind  it ;  the  prompt 
movement  of  the  needle  declares  the  passage  of  radiant 
heat  through  the  bromine.  This  consists  entirely  of  the 
obscure  rays  of  the  candle,  for  the  light,  as  I  have  stated, 
is  utterly  cut  oif.  I  remove  the  candle,  and  put  in  its  place 
our  copper  ball,  heated  not  quite  to  redness.  The  needle 
at  once  flies  to  its  stops,  showing  the  transparency  o'f  the 
bromine  to  the  heat  emitted  by  the  ball.  I  cannot  use 
iodine  in  a  solid  state,  but,  happily,  it  dissolves  in  bisulphide 
of  carbon.  I  have  here  the  densely  coloured  liquid  in  this 
glass  cell.  I  throw  the  parallel  electric  beam  upon  the  screen ; 
this  solution  of  iodine  completely  cuts  the  light  off;  but 
if  I  bring  my  pile  into  the  path  of  the  beam,  the  violence 
of  the  needle's  motion  shows  how  copious  is  the  transmis- 
sion of  the  obscure  rays.  It  is  impossible,  I  think,  to  close 
our  eyes  upon  this  convergent  evidence  that  the  free  atoms 
swing  with  ease  in  the  ether,  while  when  grouped  to  oscil- 
lating systems,  they  cause  its  waves  to  swell,  imparting  to 
it,  as  compound  molecules,  an  amount  of  motion  which  was 
quite  beyond  their  power  to  communicate,  as  long  as  they 
remained  uncombined. 

But  it  will  occur  to  you,  no  doubt,  that  lampblack, 
which  is  an  elementary  substance,  is  one  of  the  best  ab- 
sorbers and  radiators  in  nature.  Let  us  examine  this  sub- 
stance a  little :  ordinary  lampblack  contains  many  impur- 
ities ;  it  has  various  hydro-carbons  condensed  within  it,  and 
these  hydro-carbons  are  all  powerful  absorbers  and  radia- 
tors. Lampblack,  therefore,  as  hitherto  applied,  can  hardly 
be  considered  an  element  at  all.  I  have,  however,  had  these 


RADIATION   THROUGH   LAMPBLACK,  ETC.  Oil 

hydro-carbons  in  great  part  removed,  by  carrying  through 
red  hot  lampblack  a  current  of  chlorine  gas  ;  but  the  sub- 
stance has  continued  to  be  both  a  powerful  radiator  and  a 
powerful  absorber.  Well,  what  is  lampblack  ?  Chemists 
will  tell  you  that  it  is  an  allotropic  form  of  the  diamond : 
here,  in  fact,  is  a  diamond  reduced  to  charcoal  by  intense 
heat.  Now,  the  allotropic  condition  has  long  been  defined 
as  due  to  a  difference  in  the  arrangement  of  a  body's  parti- 
cles ;  hence,  it  is  conceivable  that  this  arrangement,  which 
causes  such  a  marked  physical  difference  between  lampblack 
and  diamond,  may  consist  of  an  atomic  grouping,  which 
causes  the  body  to  act  on  radiant  heat  as  if  it  were  a  com- 
pound. I  say  such  an  arrangement  of  an  element,  though 
exceptional,  is  quite  conceivable  ;  and  I  shall  show  you  this 
to  be  eminently  the  case  as  regards  an  allotropic  form  of 
our  highly  ineffectual  oxygen. 

But,  in  reality,  lampblack  is  not  so  impervious  as  you 
might  suppose  it  to  be.  Melloni  has  shown  it  to  be  trans- 
parent, in  an  unexpected  degree,  to  radiant  heat  emanating 
from  a  low  source,  and  I  have  prepared  an  experiment 
which  will  corroborate  his.  Here  is  a  plate  of  rock-salt, 
which,  by  holding  it  over  a  smoky  lamp,  has  been  so  thick- 
ly coated  with  soot  that  it  does  not  allow  a  trace  of  light 
from  the  most  brilliant  gas  jet  to  pass  through  it.  I  place 
the  plate  upon  its  stand,  and  between  it  and  this  vessel  of 
boiling  water,  which  is  to  serve  as  our  source  of  heat,  I 
place  a  screen.  The  thermo-electric  pile  is  at  the  other  side 
of  the  smoked  plate.  The  needle  is  now  at  zero,  and  I 
withdraw  my  screen ;  instantly  the  needle  moves,  and  its 
final  and  permanent  deflection  is  52°.  I  now  cleanse  the 
salt  perfectly,  and  determine  the  radiation  through  the  un- 
smoked  plate, — it  is  71°.  Now,  the  value  of  the  deflec- 
tion 52°,  expressed  with  reference  to  our  usual  unit,  is 
90,  and  the  value  of  71°,  or  the  total  radiation,  is  about 
300.  Hence,  the  radiation  through  the  soot  is  to  the  whole 
radiation  as 


372  LECTUKE   X. 

222  :  85  =  100  :  38 

that  is  to  say, '3 8  per  cent,  of  the  incident  heat  has  been 
transmitted  by  the  layer  of  lampblack. 

Iodide  of  methyl  is  formed  by  the  union  of  the  ele- 
ment iodine  with  the  radical  methyl.  Exposure  to  light 
usually  sets  a  portion  of  the  iodine  free,  and  colours  the 
liquid  a  rich  brown.  In  a  series  of  experiments  on  the 
radiation  of  heat  through  liquids,  I  compared,  as  regards 
their  powers  of  transmission,  a  strongly  coloured  speci- 
men of  the  iodide  of  methyl,  with  a  perfectly  transparent 
one ;  there  was  no  difference  between  them.  The  iodine, 
which  produced  so  marked  an  effect  on  light,  did  not  sen- 
sibly affect  radiant  heat.  Here  are  the  numbers  which 
express  the  portion  of  the  total  radiation  intercepted  by 
the  transparent  and  coloured  liquids  respectively : — 

Absorption  per  cent. 

Iodide  of  methyl  (transparent)  .  .  .     53'2 

"  "      (strongly  coloured  with  iodine)        .     53*2 

The  source  of  heat,  in  this  case,  was  a  spiral  of  platinum 
wire  raised  to  bright  redness  by  an  electric  current.  On 
looking  through  the  coloured  liquid,  the  incandescent 
spiral  was  visible.  I  therefore  intentionally  deepened  the 
colour  by  adding  iodine,  until  the  solution  was  of  suffi- 
cient opacity  to  cut  off  wholly  the  light  of  a  brilliant  jet 
of  gas.  The  transparency  of  the  liquid  to  the  radiant 
heat  was  not  sensibly  affected  by  the  addition  of  the 
iodine.  The  luminous  heat  was,  of  course,  cut  off;  but 
this,  as  compared  with  the  whole  radiation,  was  so  small 
as  to  be  insensible  in  the  experiments. 

It  is  known  that  iodine  dissolves  freely  in  the  bisul- 
phide of  carbon,  the  colour  of  the  solution  in  thin  layers 
being  a  splendid  purple ;  but  in  layers  of  moderate  thick- 
ness it  may  be  rendered  perfectly  opaque  to  light.  I  dis- 
solved a  quantity  of  iodine  in  the  liquid,  sufficient,  when 
introduced  into  a  cell  0*07  of  an  inch  wide,  to  cut  off  the 


HEAT    SPECTKUM   BY   OPAQUE   PEISMS.  373 

light  of  the  most  brilliant  gas  flame.  Comparing  the 
opaque  solution  with  the  transparent  bisulphide,  the  fol- 
lowing results  were  obtained : — 

Absorption. 

Bisulphide  of  carbon  (opaque)    .  .  .     12'5 

u       (transparent)  .     12'5 

Here  the  presence  of  a  quantity  of  iodine,  perfectly  opaque 
to  a  brilliant  light,  was  without  measurable  effect  upon 
the  heat  emanating  from  our  platinum  spiral. 

The  same  liquid  was  placed  in  a  cell  0'27  of  an  inch  in 
width;  that  is  to  say,  a  solution  which  was  perfectly 
opaque  to  light,  at  a  thickness  of  0*07,  was  employed  in  a 
layer  of  nearly  four  times  this  thickness.  Here  are  the 
results : — 

Absorption. 

Bisulphide  of  carbon  (transparent)          .  .     18'8 

"      (opaque)     .  .  .     19'0 

The  difference  between  both  measurements  lies  within  the 
limits  of  possible  error. 

I  have  already  had  occasion  to  decompose  in  your 
presence  the  light  of  the  electric  lamp,  and  to  project  the 
spectrum  of  the  light  upon  the  screen  behind  me.  For 
this  purpose,  I  employed  a  prism  of  transparent  bisulphide 
of  carbon.  The  liquid  is  contained  in  a  wedge-shaped 
flask  with  plane  glass  sides;  it  draws  the  colours  very 
widely  apart,  and  produces  a  more  beautiful  effect  than 
could  be  obtained  with  a  glass  prism.  My  object  is  now 
to  project  a  little  spectrum  on  this  small  screen.  Behind 
the  screen  I  have  placed  my  thermo-electric  pile,  which  is 
connected  with  the  large  galvanometer  in  front  of  the 
table.  The  spectrum,  as  you  observe,  is  about  1-J  inches 
wide  and  2  inches  long,  its  colours  being  rendered  very 
vivid  by  concentration.  If  I  removed  the  screen,  the  red 
and  extra-red  of  the  spectrum  would  fall  upon  the  pile 
behind,  and  doubtless  produce  a  thermo-electric  current. 
But  I  do  riot  wish  any  of  the  light  to  fall  upon  the  instru- 


374 


LECTURE   X. 


ment ;  I  wish  to  show  you  that  we  have  here  a  spectrum 
which  you  cannot  see,  and  that  you  may  entirely  detach 
the  non-luminous  spectrum  from  the  luminous  one. 
Here,  then,  is  a  second  prism,  filled  with  the  bisulphide  of 
carbon,  in  which  iodine  has  been  dissolved.  I  remove  the 
transparent  prism,  and  put  the  opaque  one  exactly  in  its 
place.  The  spectrum  has  disappeared ;  there  is  no  longer 
a  trace  of  light  upon  the  screen;  but  a  thermal  spectrum 
is  still  there.  The  obscure  rays  of  the  electric  lamp  have 
traversed  the  opaque  liquid,  have  been  refracted  like  the 
luminous  ones,  and  are  now,  though  invisible,  impinging 
upon  the  screen  before  you.  I  prove  this,  by  removing 
the  screen :  no  light  strikes  the  pile,  but  you  see  that  the 
heat  falling  upon  it  is  competent  to  dash  violently  aside 
the  needles  of  our  large  galvanometer. 

I  have  shown  you  the  action  of  gases  upon  radiant 
heat,  with  our  glass  experimental  tube  and  our  new  source 
of  heat.  Let  me  now  refer  to  the  action  of  vapours,  as 
examined  with  the  same  apparatus.  Here  I  have  several 
glass  flasks,  each  furnished  with  a  brass  cap,  to  which  a 
stopcock  can  be  screwed.  Into  each  I  pour  a  quantity  of 
a  volatile  liquid,  reserving  a  flask  for  each  liquid,  so  as  to 
render  admixture  of  the  vapours  impossible.  From  each 
flask  I  remove  the  air, — not  only  the  air  above  the  liquid, 
but  the  air  dissolved  in  it.  This  latter  bubbles  freely 
away  when  the  flask  is  exhausted ;  I  now  attach  my  flask 
to  the  exhausted  experimental  tube,  and  allow  the  vapour 
to  enter,  without  permitting  any  ebullition  to  occur.  The 
mercury  column  of  the  pump  sinks,  and  when  the  re- 
quired depression  has  been  obtained,  I  cut  off  the  supply 
of  vapour.  In  this  way,  the  vapours  of  the  substances 
mentioned  in  the  next  table  have  been  examined,  at  pres- 
sures of  O'l,  0-5,  and  1  inch,  respectively. 


ACTION   OF   VAPOTJES   ON   KADIANT   HEAT.  375 

Absorption  of  Vapours 
at  the  pressures 

0-1  0-5  1-0 

Bisulphide  of  carbon  .  .  .       15  47  62 

Iodide  of  methyl  .  .  .35  147  242 

Benzol   .             .  .  .  .       66  182  267 

Chloroform         .  .  .  .  -    85  182  236 

Methylic  alcohol  .  .  .     109  390  590 

Amylene             .  .  .  .     182  535  822 

Sulphuric  ether  .  .  .300  710  870 

Alcohol               .  .  .  .325  622 

Formic  ether      .  .  .  .480  870  1075 

Acetic  ether       .  .  .  .590  960  1195 

Propionate  of  ethyl  .  .  .596  970 

Boracicacid        .  .  .  .620 

These  numbers  refer  to  the  absorption  of  a  whole  at- 
mosphere of  dry  air  as  their  unit ;  that  is  to  say,  -^ih  of 
an  inch  of  bisulphide  of  carbon  vapour  does  fifteen  times 
the  execution  of  30  inches  of  atmospheric  air ;  while  -fVth 
of  an  inch  of  boracic  ether  vapour  does  620  times  the 
execution  of  a  whole  atmosphere  of  atmospheric  air. 
Comparing  air  at  a  pressure  of  0*01  with  boracic  ether  at 
the  same  pressure,  the  absorption  of  the  latter  is  probably 
180,000  times  that  of  the  former. 


NOTE. 

(7)  A  wave  of  ether  starting  from  a  radiant  point  in  all  directions,  in 
a  uniform  medium,  constitutes  a  spherical  shell,  which  expands  with  the 
velocity  of  light  or  of  radiant  heat.  A  ray  of  light,  or  a  ray  of  heat,  is  a 
line  perpendicular  to  the  wave,  and,  in  the  case  here  supposed,  the  raya 
would  be  the  radii  of  the  spherical  shell.  The  word  '  ray,'  however,  is 
used  in  the  text,  to  avoid  circumlocution,  as  equivalent  to  the  term  unii 
of  heat.  Thus,  calling  the  amount  of  heat  intercepted  by  a  whole  atmos- 
phere of  air  1,  the  amount  intercepted  by  ^th  of  an  atmosphere  of  de- 
fiant gas  is  90. 


APPENDIX    TO    LECTUPvE    X. 


I  GIVE  here  the  method  of  calibrating  the  galvanometer  which 
Melloni  recommends,  as  leaving  nothing  to  be  desired  as  regards 
facility,  promptness,  and  precision.  His  own  statement  of  the' 
method,  translated  from  La  Thermochrose,  page  59,  is  as 
follows : — 

Two  small  vessels,  v  v,  are  half-  rig.  93. 

filled  with  mercury,  and  connected, 
separately,  by  twro  short  wires,  with 
the  extremities  G  G  of  the  galvano- 
meter. The  vessels  and  wires  thus 
disposed  make  no  change  in  the  ac- 
tion of  the  instrument ;  the  thermo- 
electric current  being  freely  trans- 
mitted, as  before,  from  the  pile  to 
the  galvanometer.  But  if,  by 
means  of  a  wire  F,  a  communication 
be  established  between  the  two  vessels,  part  of  the  current  will 
pass  through  this  wire  and  return  to  the  pile.  The  quantity  of 
electricity  circulating  in  the  galvanometer  will  be  thus  dimin- 
ished and  with  it  the  deflection  of  the  needle. 

Suppose,  then,  that  by  this  artifice  we  have  reduced  the  gal- 
vanometric  deviation  to  its  fourth  or  fifth  part ;  in  other  words, 
supposing  that  the  needle  being  at  10  or  12  degrees,  under  the 
action  of  a  constant  source  of  heat,  placed  at  a  fixed  distance 
from  the  pile,  that  it  descends  to  2  or  3  degrees  wrhen  a  portion 
of  the  current  is  diverted  by  the  external  wire ;  I  say  that  by 
causing  the  source  to  act  from  various  distances,  and  observing 
%  each  case  the  total  deflection,  and  the  reduced  deflection,  WT<? 


CALIBRATION   OF  GALVANOMETER.  377 

have  all  the  data  necessary  to  determine  the  ratio  of  the  deflec- 
tions of  the  needle,  to  the  forces  which  produce  these  deflections. 

To  render  the  exposition  clearer,  and  to  furnish,  at  the  same 
time,  an  example  of  the  mode  of  operation,  I  will  take  the  num- 
bers relating  to  the  application  of  the  method  to  one  of  my 
thermo-multipliers. 

The  external  circuit  being  interrupted,  and  the  source  of  heat 
being  sufficiently  distant  from  the  pile  to  give  a  deflection  not 
exceeding  5  degrees  of  the  galvanometer,  let  the  wire  be  placed 
from  v  to  v;  the  needle  falls  to  1°*5.  The  connection  between 
the  two  vessels  being  again  interrupted,  let  the  source  be  brought 
near  enough  to  obtain  successively  the  deflections : — 
5°,  10°,  15°,  20°,  25°,  30°,  35°,  40°,  45°. 

Interposing  after  each  the  same  wire  between  v  and  v  we  ob- 
tain the  following  numbers  : — 

r-5,  3°,  4°-5,  G°-3,  8°-4,  ll°-2,  15°'3,  22°-4,  29°-7. 

Assuming  the  force  necessary  to  cause  the  needle  to  describe 
each  of  the  first  degrees  of  the  galvanometer  to  be  equal  to  unity, 
we  have  the  number  5  as  the  expression  of  the  force  corresponding 
to  the  first  observation.  The  other  forces  are  easily  obtained  by 
the  proportions : — 

1-5:  6=a:  x=i?s-  a=3'333  a* 

where  a  represents  the  deflection  when  the  exterior  circuit  is 
closed.    "We  thus  obtain 

5,  10,  15-2,  21,  28,  37-3. 

for  the  forces,  corresponding  to  the  deflections, 
5°,  10°,  15°,  20°,  25°,  30°. 

In  this  instrument,  therefore,  the  forces  are  sensibly  propor- 
tional to  the  arcs,  up  to  nearly  15  degrees.  Beyond  this,  the  pro- 
portionality ceases,  and  the  divergence  augments  as  the  arcs  in- 
crease in  size. 

The  forces  belonging  to  the  intermediate  degrees  are  obtained 
with  great  ease  either  by  calculation  or  by  graphical  construc- 
tion, which  latter  is  sufficiently  accurate  for  these  determinations. 

*  That  is  to  say,  one  reduced  current  is  to  the  total  current  to  which 
it  corresponds,  as  any  other  reduced  current  is  to  its  corresponding  total 
current. 


378  APPENDIX   TO   LECTURE   X. 

By  these  means  we  find, 

Degrees    .'       .     13°,  H°,  15°,  16°,  17°,  18°,  19°,  20°,  21°. 
Forces       .         .     13,  14-1,  15-2,  16'3,  17'4,  18'6,  19-8,  21,  22-3. 
Differences        .     1-1,  1-1,  1-1,  1-1,  1.2,  1-2,  1-2,  1-3. 
Degrees     .         .     22°,  23°,  24°,  25°,  26°,  27°,  28°,  29°,  30°. 
Forces       .         .     23-5,  24'9,  26-4,  28,  29'7,  31-5,  33'4,  35'3,  37'3. 
Differences        .     1-4,  1-5,  1-6,  1'7,  1-8,  1-9,  2. 
In  this  table  we  do  not  take  into  account  any  of  the  degrees 
preceding  the  13th,  because  the  force  corresponding  to  each  of 
them  possesses  the  same  value  as  the  deflection. 

The  forces  corresponding  to  the  first  30  degrees  being  known, 
nothing  is  easier  than  to  determine  the  values  of  tlie  forces  corre- 
sponding to  35,  40,  45  degrees,  and  upwards. 
The  reduced  deflections  of  these  three  arcs  are, 

15°'3,  22°-4,  29°-7. 

Let  us  consider  them  separately ;  commencing  with  the  first. 
In  the  first  place,  then,  15  degrees,  according  to  our  calculation, 
are  equal  to  15'2  ;  we  obtain  the  value  of  the  decimal  0'3  by  mul- 
tiplying this  fraction  by  the  difference  1-1  which  exists  between 
the  15th  and  16th  degrees ;  for  we  have  evidently  the  proportion 

1:  1-1=0-3  :x=0-3. 

The  value  of  the  reduced  deflection  corresponding  to  the  35th 
degree  will  not,  therefore,  be  15°-3,  but  15°-2  +  0°-3=15°-5.  By 
similar  considerations  we  find  230'5  +  00'6=24°-1,  instead  of 
22°-4,  and  36°'7  instead  of  29°'7  for  the  reduced  deflections  of  40 
and  45  degrees. 

It  now  only  remains  to  calculate  the  forces  belonging  to  these 
three  deflections,  15°-5,  24°'l,  and  36°'7,  by  means  of  the  expres- 
sion 3'333  a  •  this  gives  us, 

the  forces,  51 '7,  80-3,  122-3. 
for  the  degrees,  35,  40,  45. 

Comparing  these  numbers  with  those  of  the  preceding  table, 
we  see  that  the  sensitiveness  of  our  galvanometer  diminishes  con- 
siderably when  we  use  deflections  greater  than  30  degrees. 


LECTURE    XI. 

[April  3,  1862.] 


ACTION  OF  ODOROUS  SUBSTANCES  UPON  RADIANT  HEAT — ACTION  OP  OZONE 
UPON  RADIANT  HEAT — DETERMINATION  OF  THE  RADIATION  AND  ABSORP- 
TION OF  GASES  AND  YAPOURS  WITHOUT  ANY  SOURCE  OF  HEAT  EXTERNAL 
TO  THE  GASEOUS  BODY — DYNAMIC  RADIATION  AND  ABSORPTION — RADIA- 
TION THROUGH  THE  EARTH'S  ATMOSPHERE — INFLUENCE  OF  THE  AQUEOUS 
VAPOUR  OF  THE  ATMOSPHERE  ON  RADIANT  HEAT— CONNECTION  OF  THE 
RADIANT  AND  ABSORBENT  POWER  OF  AQUEOUS  VAPOUR  WITH  METEORO- 
LOGICAL PHENOMENA. 


APPENDIX  :  FURTHER  DETAILS  OF  THE  ACTION  OF  HUMID  AIR. 

SCENTS  and  effluvia  generally  have  long  occupied  the 
attention  of  observant  men,  and  they  have  formed  fa- 
vourite illustrations  of  the  '  divisibility  of  matter.'  No 
chemist  ever  weighed  the  perfume  of  a  rose ;  but  in  ra- 
diant heat  we  have  a  test  more  refined  than  the  chemist's 
balance.  The  results  brought  before  you  in  our  last  lecture 
would  enable  you  to  hear  me  without  surprise,  were  I  to 
assert  that  the  quantity  of  volatile  matter  removed  from  a 
hartshorn  bottle  by  any  person  in  this  room,  by  a  single  act 
of  inhalation,  would  exercise  a  more  potent  action  on  ra- 
diant heat,  than  the  whole  body  of  oxygen  and  nitrogen 
which  the  room  contains.  Let  us  apply  this  test  to  other 
odours,  and  see  whether  they  also,  notwithstanding  their 
almost  infinite  attenuation,  do  not  exercise  a  measurable  in- 
fluence on  radiant  heat. 

I  will  operate  in  this  simple  way  :  here  is  a  number  of 
small  and  equal  squares  of  bibulous  paper,  which  I  roll  up 
thus,  to  form  little  cylinders,  each  about  two  inches  in 
length.  I  moisten  the  paper  cylinder  by  dipping  one  end 


380  LECTURE   XI. 

of  it  into  an  aromatic  oil ;  the  oil  creeps  by  capillary  attrac- 
tion through  the  paper,  and  the  whole  of  the  cylinder  is 
now  moist.  I  introduce  the  rolled  paper  thus  into  a  glass 
tube  of  such  a  diameter  that  the  cylinder  fills  it  without 
being  squeezed,  and  between  my  drying  apparatus  and  the 
experimental  cylinder  I  place  the  tube  containing  the 
scented  paper.  The  experimental  cylinder  is  now  exhaust- 
ed, and  the  needle  at  zero ;  turning  this  cock  on,  I  allow 
dry  air  to  pass  gently  through  the  folds  of  the  saturated 
paper.  Here  the  air  takes  up  the  perfume  of  the  aromatic 
oil,  and  carries  it  into  the  experimental  tube.  The  absorp- 
tion of  an  atmosphere  of  dry  air  we  know  to  be  unity ;  it 
produces  a  deflection  of  one  degree  ;  hence,  any  additional 
absorption  which  these  experiments  reveal,  must  be  due  to 
the  scent  which  accompanies  the  air. 

The  following  table  will  give  a  condensed  view  of  the 
absorption  of  the  substances  mentioned  in  it ;  air  at  the 
tension  of  one  atmosphere  being  regarded  as  unity : — 

Perfumes. 

Name  of  Perfume  Absorption 

Pachouli 30 

Sandal  Wood          ....  32 

Geranium 33 

Oil  of  Cloves          .        .        .        .  33-5 

Otto  of  Roses         ....  36-5 

Bergamot 44 

Neroli             47 

Lavender 60 

Lemon           .                 ...  65 

Portugal 67 

Thyme 68 

Rosemary 74 

Oil  of  Laurel 80 

Camomile  Flowers  ...  87 

Cassia 109 

Spikenard 355 

Aniseed  372 


ACTION  OF   SCENTS  ON  KADIANT  HEAT.  381 

The  number  of  atoms  of  air  here  in  the  tube  must  be 
regarded  as  almost  infinite  in  comparison  with  those  of  the 
odours  ;  still  the  latter,  thinly  scattered  as  they  are,  do,  in 
the  case  of  pachouli,  30  times  the  execution  of  the  air  ;  otto 
of  roses  does  upwards  of  36  times  the  execution  of  the 
air ;  thyme,  74  times ;  spikenard,  355  times ;  and  aniseed 
3*72  times  the  execution  of  the  air.  It  would  be  idle  to 
speculate  on  the  quantities  of  matter  implicated  in  these 
results.  Probably  they  would  have  to  be  multiplied  by 
millions  to  bring  them  up  to  the  tension  of  ordinary  air. 
Thus,— 

The  sweet  south 

That  breathes  upon  a  bank  of  violets, 

Stealing  and  giving  odour, 

owes  its  sweetness  to  an  agent,  which,  though  almost  infi- 
nitely attenuated,  may  be  more  potent,  as  an  intercepter  of 
terrestrial  radiation,  than  the  entire  atmosphere  from 
'  bank '  to  sky. 

-  In  addition  to  these  experiments  on  the  essential  oils, 
others  were  made  on  aromatic  herbs.  A  number  of  such 
were  obtained  from  Covent  Garden  Market;  they  were 
dry,  in  the  common  acceptation  of  the  term ;  that  is  to 
say,  they  were  not  green,  but  withered.  Still  I  fear  the 
results  obtained  with  them  cannot  be  regarded  as  pure,  on 
account  of  the  probable  admixture  of  aqueous  vapour.  The 
aromatic  parts  of  the  plants  were  stuffed  into  a  glass  tube 
eighteen  inches  long  and  a  quarter  of  an  inch  in  diameter. 
Previous  to  connecting  them  with  the  experimental  tube, 
they  were  attached  to  a  second  air-pump,  and  dry  air  was 
carried  over  them  for  some  minutes.  They  were  then  con- 
nected with  the  experimental  cylinder,  and  treated  as  the 
essential  oils ;  the  only  difference  being  that  a  length  of 
eighteen  inches,  instead  of  two,  was  occupied  by  the  herbs. 
Thyme,  thus  examined,  gave  an  action  thirty-three  times 
that  of  the  air  which  passed  over  it. 


382  LECTURE   XI. 

Peppermint  exercised  thirty-four  tunes  the  action  of  the 
air. 

Spearmint  exercised  thirty-eight  times  the  action  of  the 
air. 

Lavender  exercised  thirty-two  times  the  action  of  the 
air. 

Wormwood  exercised  forty-one  times  the  action  of  the 
air. 

Cinnamon  exercised  fifty-three  times  the  action  of  the 
air. 

As  already  hinted,  I  fear  that  these  results  may  be 
complicated  with  the  action  of  aqueous  vapour  :  its  quan- 
tity, however,  must  have  been  infinitesimal. 

There  is  another  substance  of  great  interest  to  the 
chemist,  but  the  attainable  quantities  of  which  are  so  mi- 
nute as  almost  to  elude  measurement,  to  which  we  may  ap- 
ply the  test  of  radiant  heat.  I  mean  that  extraordinary 
substance,  ozone.  This  body  is  known  to  be  liberated  at 
the  oxygen  electrode,  when  water  is  decomposed  by  an 
electric  current.  To  investigate  its  action  I  had  construct- 
ed three  different  decomposing  cells.  In  the  first,  which  I 
shall  call  ETo.  1,  the  platinum  plates  used  as  electrodes  had 
about  four  square  inches  of  surface ;  the  plates  of  the  sec- 
ond (No.  2)  had  two  square  inches  of  surface ;  while  the 
plates  of  the  third  (No.  3)  had  only  one  square  inch  of  sur- 
face, each. 

My  reason  for  using  electrodes  of  different  sizes  was 
this : — On  first  applying  radiant  heat  to  the  examination  of 
ozone,  I  constructed  a  decomposing  cell,  in  which,  to  di- 
minish the  resistance  of  the  current,  very  large  platinum 
plates  were  used.  The  oxygen  thus  obtained,  and  wThich 
ought  to  have  embraced  the  ozone,  showed  scarcely  any  of 
the  reactions  of  this  substance.  It  hardly  discoloured 
iodide  of  potassium,  and  wras  almost  without  action  on  ra- 
diant heat.  A  second  decomposing  apparatus,  with  smaller 


ACTION   OF   OZONE   ON  RADIANT   HEAT.  383 

plates,  was  tried,  and  here  I  found  both  the  action  on  iodide 
of  potassium,  and  on  radiant  heat,  very  decided.  Being 
unable  to  refer  these  differences  to  any  other  cause  than  the 
different  magnitudes  of  the  plates,  I  formally  attacked  the 
subject  by  operating  with  the  three  cells  above  described. 
Calling  the  action  of  the  main  body  of  the  electrolytic  oxy- 
gen unity ;  that  of  the  ozone  which  accommpanied  it,  in 
the  respective  cases,  is  given  in  the  following  table  : — 

Number  of  Cell  Absorption. 

No.  1 20 

No.  2 34 

No.  3 47 

Thus  the  modicum  of  ozone  which  accompanied  the 
oxygen,  and  in  comparison  to  which  it  is  a  vanishing  quan- 
tity, exerted,  in  the  case  of  the  first  pair  of  plates,  an  action 
twenty  times  that  of  the  oxygen  itself,  while  with  the  third 
pair,  of  plates  the  ozone  was  forty-seven  times  more  ener- 
getic than  the  oxygen.  The  influence  of  the  size  of  the 
plates,  or,  in  other  words,  of  the  density  of  the  current 
where  it  enters  the  liquid,  on  the  production  of  the  ozone, 
is  rendered  strikingly  manifest  by  these  experiments. 

I  then  cut  away  portions  of  the  plates  of  cell  No.  2,  so 
as  to  make  them  smaller  than  those  of  No.  3.  The  reduc- 
tion of  the  plates  was  accompanied  by  an  augmentation  of 
the  action  upon  radiant  heat ;  the  absorption  rose  at  once 
from  34  to 

65. 

The  reduced  plates  of  No.  2  here  transcend  those  of 
No.  3,  which,  in  the  first  experiments,  gave  the  largest 
action. 

The  plates  of  No.  3  were  next  reduced,  so  as  to  make 
them  smallest  of  all.  The  ozone  now  generated  by  No.  3, 
effected  an  absorption  of 

85. 


384:  LECTUEE  XI. 

Thus  we  see  that  the  action  upon  radiant  heat  advances 
as  the  size  of  the  electrodes  is  diminished. 

Heat  is  known  to  be  very  destructive  of  ozone,  and 
suspecting  the  developement  of  heat  at  the  small  electrodes 
of  the  cell  last  made  use  of,  I  surrounded  the  cell  with  a 
mixture  of  pounded  ice  and  salt.  Kept  thus  cool,  the  ab- 
sorption of  the  ozone  generated  rose  to 

136. 

These  experiments  on  the  action  of  ozone  upon  radiant 
heat  were  made,  before  I  was  acquainted  with  the  re- 
searches of  MM.  De  la  Rive,  Soret,  and  Meidinger,  on  this 
substance.  There  is  a  perfect  correspondence  in  our  re- 
sults, though  there  is  no  resemblance  between  our  modes 
of  experiment.  Such  a  correspondence  is  calculated  to 
augment  our  confidence  in  radiant  heat,  as  an  investigator 
of  molecular  condition.* 

*  M.  Meidinger  commences  his  paper  by  showing  the  absence  of  agree- 
ment between  theory  and  experiment  in  the  decomposition  of  water,  the 
difference  showing  itself  very  decidedly  in  a  deficiency  of  oxygen  when  the 
current  was  strong.  On  heating  his  electrolyte,  he  found  that  this  dif- 
ference disappeared,  the  proper  quantity  of  oxygen  being  then  liberated. 
He  at  once  surmised  that  the  defect  of  oxygen  might  be  due  to  the  forma- 
tion of  ozone  ;  but  how  did  the  substance  act  to  produce  the  diminution  of 
the  oxygen  ?  If  the  defect  were  due  to  the  great  density  of  the  ozone,  the 
destruction  of  this  substance,  by  heat,  would  restore  the  oxygen  to  its  true 
volume.  Strong  heating,  however,  which  destroyed  the  ozone,  produced 
no  alteration  of  volume,  hence  M.  Meidinger  concluded  that  the  effect 
which  he  observed  was  not  due  to  the  ozone  which  remained  mixed  with 
the  oxygen  itself.  He  finally  concluded,  and  justified  his  conclusion  by 
satisfactory  experiments,  that  the  loss  of  oxygen  was  due  to  the  formation 
in  the  water,  of  peroxide  of  hydrogen  by  the  ozone  ;  the  oxygen  being  thus 
withdrawn  from  the  tube  to  which  it  belonged.  He  also,  as  M.  De  la  Rive 
had  previously  done,  experimented  with  electrodes  of  different  sizes,  and 
found  the  loss  of  oxygen  much  more  considerable  when  a  small  electrode 
was  used  than  with  a  large  one  ;  whence  he  inferred  that  the  formation  of 
ozone  was  facilitated  by  augmenting  the  density  of  the  current  at  the  place 
where  electrode  and  electrolyte  meet.  The  same  conclusion  is  deduced  from 


CONSTITUTION   OF   OZONE.  385 

The  quantities  of  ozone  with  which  the  foregoing  ex- 
periments were  made,  must  be  perfectly  immeasurable  by 
ordinary  means.  Still  its  action  upon  radiant  heat  is  so 
energetic,  as  to  place  it  beside  olefiant  gas,  or  boracic  ether, 
as  an  absorbent — bulk  for  bulk  it  might  transcend  either. 
IsTo  elementary  gas  that  I  have  examined  behaves  at  all  like 
ozone.  In  its  swing  through  the  ether  it  must  powerfully 
disturb  the  medium.  If  it  be  oxygen,  it  must,  I  think,  be 
oxygen  atoms  packed  into  groups.  I  sought  to  decide  the 
question  whether  it  is  oxygen,  or  a  compound  of  hydrogen, 
in  the  following  way.  Heat  destroys  ozone.  If  it  were 
oxygen  only,  heat  would  convert  it  into  the  common  gas  ; 
if  it  were  the  hydrogen  compound,  which  some  chemists 
consider  it  to  be,  heat  would  convert  it  into  oxygen,  plus 
aqueous  vapour.  The  gas  alone,  admitted  into  my  tube, 
would  give  the  neutral  action  of  oxygen,  but  the  gas,  plus 
the  aqueous  vapour,  I  hoped  might  give  a  sensibly  greater 
action.  The  dried  electrolytic  gas  was  caused  to  pass 
through  a  glass  tube  heated  to  redness,  and  thence  direct 
into  the  experimental  tube.  It  was  next,  after  heating, 
made  to  pass  through  a  drying  tube  into  the  experimental 
tube.  Hitherto  I  have  not  been  able  to  establish,  with  cer- 
tainty, a  difference  between  the  dried  and  undried  gas.  If, 
therefore,  the  act  of  heating  develope  aqueous  vapour,  the 
experimental  means  which  I  have  employed  have  not  yet 
enabled  me  to  detect  it.  For  the  present,  therefore,  I  hold 


the  above  experiments  on  radiant  heat.  No  two  things  could  be  more  di- 
verse than  the  two  modes  of  proceeding.  M.  Meidinger  sought  for  the 
oxygen  which  had  disappeared,  and  found  it  in  the  liquid ;  I  examined  the 
oxygen  actually  liberated,  and  found  that  the  ozone  mixed  with  it  aug- 
ments in  quantity  as  the  electrodes  diminish  in  size.  It  may  be  added  that 
since  the  perusal  of  M.  Meidinger's  paper  I  have  repeated  his  experiments 
with  my  own  decomposition  cells,  and  found  that  those  which  gave  me  the 
greatest  absorption,  also  showed  the  greatest  deficiency  in  the  amount  of 
oxygen  liberated. 

17 


386  LECTURE   XT. 

the  belief,  that  ozone  is  produced  by  the  packing  of  the 
atoms  of  elementary  oxygen  into  oscillating  groups ;  and 
that  heating  dissolves  the  bond  of  union,  and  allows  the 
atoms  to  swing  singly,  thus  disqualifying  them  for  either 
intercepting  or  generating  the  motion,  which,  as  systemSj 
they  are  competent  to  intercept  and  generate. 

I  have  now  to  direct  your  attention  to  a  series  of  facts 
which  surprised  and  perplexed  me  when  I  first  observed 
them.  While  experimenting  last  November  (1861),  on  one 
occasion  I  permitted  a  quantity  of  alcohol  vapour,  sufficient 
to  depress  the  mercury  gauge  0*5  of  an  inch,  to  enter  the 
experimental  tube  ;  it  produced  a  deflection  of  72°.  While 
the  needle  pointed  to  this  high  figure,  and  previously  to 
pumping  out  the  vapour,  I  allowed  dry  air  to  stream  into 
the  tube,  and  happened,  as  it  entered,  to  keep  my  eye  upon 
the  galvanometer. 

The  needle,  to  my  astonishment,  sank  speedily  to  zero, 
and  went  to  25°  on  the  opposite  side.  The  entry  of  the 
almost  neutral  air,  not  only  neutralised  the  absorption  pre- 
viously observed,  but  left  a  considerable  balance  in  favour 
of  the  face  of  the  pile  turned  towards  the  source.  A  repe- 
tition of  the  experiment  brought  the  needle  down  from  70° 
to  zero,  and  sent  it  to  38°  on  the  opposite  side.  In  like 
manner,  a  very  small  quantity  of  the  vapour  of  sulphuric 
ether  produced  a  deflection  of  30°  ;  on  allowing  dry  air  to 
fill  the  tube,  the  needle  descended  speedily  to  zero,  and 
swung  to  60°  at  the  opposite  side. 

My  first  thought,  on  observing  these  extraordinary 
effects,  was,  that  the  vapours  had  deposited  themselves  in 
opaque  films  on  the  plates  of  rock-salt,  and  that  the  dry  air 
on  entering  had  cleared  these  films  away,  and  allowed  the 
heat  from  the  source  free  transmission. 

But  a  moment's  reflection  dissipated  this  supposition. 
The  clearing  away  of  such  a  film  could,  at  best,  but  restore 
the  state  of  things  existing  prior  to  the  entrance  of  the 


DYNAMIC  RADIATION.  387 

vapour.  It  might  be  conceived  to  bring  the  needle  again  to 
0°,  but  it  could  not  possibly  produce  the  negative  deflec- 
tion. Nevertheless,  I  dismounted  the  tube,  and  subjected 
the  plates  of  salt  to  a  searching  examination.  No  such  de- 
posit as  that  above  surmised  was  observed.  The  salt  re- 
mained perfectly  transparent  while  in  contact  with  the 
vapour.  How,  then,  are  the  effects  to  be  accounted  for  ? 

We  have  already  made  ourselves  acquainted  with  the 
thermal  effects  produced  when  air  is  permitted  to  stream 
into  a  vacuum  (page  44).  We  know  that  the  air  is 
warmed  by  its  collision  against  the  sides  of  the  receiver. 
Can  it  be  the  heat  thus  generated,  imparted  by  the  air  to 
the  alcohol  and  ether  vapours,  and  radiated  by  them  against 
the  pile,  that  was  more  than  sufficient  to  make  amends  for 
the  absorption?  The  experimentum  crucis  at  once  sug- 
gests itself  here.  If  the  effects  observed  be  due  to  the 
heating  of  the  air  on  entering  the  partial  vacuum  in  which 
the  vapour  was  diffused,  we  ought  to  obtain  the  same 
effects  when  the  sources  of  heat  made  use  of  hitherto  are 
entirely  abolished.  We  are  thus  led  to  the  consideration  of 
the  novel  and  at  first  sight  utterly  paradoxical  problem — 
namely,  to  determine  the  radiation  and  absorption  of  a  gas 
or  vapour  without  any  source  of  heat  external  to  the  gaseous 
lody  itself. 

Let  us,  then,  erect  our  apparatus,  and  omit  our  two 
sources  of  heat.  Here  is  our  glass  tube,  stopped  at  one  end 
by  a  plate  of  glass,  for  we  do  not  now  need  the  passage  of 
the  heat  through  this  end  ;  and  at  the  other  end  by  a  plate 
of  rock-salt.  In  front  of  the  salt  is  placed  the  pile,  con- 
nected with  its  galvanometer.  Though  there  is  now  no 
special  source  of  heat  acting  upon  the  pile,  you  see  the  nee- 
dle does  not  come  quite  to  zero  ;  indeed,  the  walls  of  this 
room,  and  the  people  who  sit  before  me,  are  so  many 
sources  of  heat,  to  neutralise  which,  and  thus  to  bring  the 
needle  accurately  to  zero,  I  must  slightly  warm  the  defect- 


388  LECTURE   XI. 

ive  face  of  the  pile.  This  is  done  without  any  difficulty  by 
a  cube  of  lukewarm  water,  placed  at  a  distance  ;  the  needle 
is  now  at  zero. 

The  experimental  tube  being  exhausted,  I  allow  air  to 
enter,  till  the  tube  is  filled ;  the  horizontal  column  of  air  at 
present  in  the  tube  is  warmed  ;  every  atom  of  the  air  is  os- 
cillating ;  and  if  the  atoms  possessed  any  sensible  power  of 
communicating  their  motion  to  the  luminiferous  ether,  we 
should  have  from  each  atom  a  train  of  waves  impinging  on 
the  face  of  the  pile.  But  you  observe  scarcely  any  motion 
of  the  galvanometer,  and  hence  may  infer  that  the  quantity 
of  heat  radiated  by  the  air  is  exceedingly  small .  The  de- 
flection produced  is  7  degrees. 

But  these  7°  are  not  really  due  to  the  radiation  of  the 
air.  To  what,  then  ?  I  open  one  of  the  ends  of  the  ex- 
perimental tube,  and  place  a  bit  of  black  paper  as  a  lining 
within  it ;  the  paper  merely  constitutes  a  ring  which  covers 
the  interior  surface  of  the  tube  for  a  length  of  12  inches. 
I  close  the  tube  and  repeat  the  last  experiment.  The  tube 
has  been  exhausted  and  the  air  is  now  entering,  but  mark 
the  needle — it  has  already  flown  through  an  arc  of  70°. 
You  see  here  exemplified  the  influence  of  this  bit  of  paper 
lining  ;  it  is  warmed  by  the  air,  and  it  radiates  towards  the 
pile  in  this  copious  way.  The  interior  surface  of  the  tube 
itself  must  da  the  same,  though  in  a  less  degree,  and  to 
the  radiation  from  this  surface,  and  not  from  the  air  itself, 
the  deflection  of  7°  which  we  have  just  obtained  is,  I  be- 
lieve, to  be  ascribed. 

Removing  the  bit  of  lining  from  the  tube,  instead  of 
air  I  allow  nitrous  oxide  to  stream  into  it ;  the  needle 
swings  to  28°,  thus  showing  the  superior  radiative  power 
of  this  gas.  I  now  work  the  pump,  the  gas  within  the  ex. 
perimental  tube  becomes  chilled,  and  into  it  the  pile  pours 
its  heat ;  a  swing  of  20°  in  the  opposite  direction  is  the  con- 
sequence. 


DYNAMIC   KADIATION   OF   GASES.  389 

Instead  of  nitrous  oxide,  I  now  allow  olefiant  gas  to 
stream  into  the  exhausted  tube.  We  have  already  learned 
that  this  gas  is  highly  gifted  with  the  power  of  radiation. 
Its  atoms  are  here  warmed,  and  everyone  of  them  asserts 
its  power  ;  the  needle  swings  through  an  arc  of  67°.  Let 
it  waste  its  heat,  and  let  the  needle  come  to  zero.  I  now 
pump  out,  and  the  consequent  chilling  of  the  gas,  within 
the  tube,  produces  a  deflection  of  40°  on  the  side  of  cold. 
We  have  certainly  here  a  key  to  the  solution  of  the  enig- 
matical effects  observed  with  the  alcohol  and  ether  vapour. 

For  the  sake  of  convenience  we  may  call  the  heating  of 
the  gas  on  entering  the  vacuum  dynamic  heating  ;  its  radi- 
ation I  have  called  dynamic  radiation,  and  its  absorption, 
when  chilled  by  pumping  out,  dynamic  absorption.  These 
terms  being  understood,  the  following  table  explains  itself. 
In  each  case  the  extreme  limit  to  which  the  needle  swung, 
on  the  entry  of  the  gas  into  the  experimental  tube,  is  re- 
corded. 

Dynamic  Radiation  of  Gases. 

Name  Limit  of  1st  impulsion 

Air  ....         7 

Oxygen  ....  7 
Hydrogen ....  7 
Nitrogen  ....  7 
Carbonic  Oxide  .  .  .19 

Carbonic  acid        .  .  .21 

Nitrous  oxide         .  .  •       31 

Olefiant  gas  .  .  .63 

We  observe  that  the  order  of  the  radiative  powers,  de- 
termined in  this  novel  way,  is  the  same  as  that  already  ob- 
tained from  a  totally  different  mode  of  experiment.  It 
must  be  borne  in  mind  that  the  discovery  of  dynamic  radi- 
ation is  quite  recent,  and  that  the  conditions  of  perfect  ac- 
curacy have  not  yet  been  developed ;  it  is,  however,  cer- 


390 


LECTURE   XI. 


tain,  that  the  mode  of  experiment  is  susceptible  of  the  last 
degree  of  precision. 

Let  us  now  turn  to  our  vapours,  and  while  dealing  with 
them  I  shall  endeavour  to  unite  two  effects  which,  at  first 
sight,  might  appear  utterly  incongruous.  We  have  already 
learned  that  a  polished  metal  surface  emits  an  extremely 
feeble  radiation  ;  but  that  when  the  same  surface  is  coated 
with  varnish  the  radiation  is  copious.  In  the  communica- 
tion of  motion  to  the  ether  the  atoms  of  the  metal  need  a 
mediator,  and  this  they  find  in  the  varnish.  They  commu- 
nicate their  motion  to  the  molecules  of  the  varnish,  and  the 
latter  are  so  related  to  the  luminiferous  ether*  that  they 


can  communicate  their  motion  to  it.     You  may  varnish  a 
metallic  surface  by  a  film  of  a  powerful  gas.    I  have  here 

*  If  we  could  change  either  the  name  given  to  the  interstellar  medium, 
or  that  given  to  certain  volatile  liquids  by  chemists,  it  would  be  an  advan- 
tage. It  is  difficult  to  avoid  confusion  in  the  use  of  the  same  name  for 
objects  so  utterly  diverse. 


DYNAMIC   RADIATION   OF   VAPOTTBS.  391 

an  arrangement  which  enables  me  to  cause  a  thin  stratum 
of  olefiant  gas  from  the  gasholder  G  (fig.  94)  to  pass  through 
the  slit  tube  a  #,  and  over  the  heated  surface  of  the  cube  c. 
The  radiation  from  c  is  now  neutralised  by  that  from  c' ; 
but  I  allow  the  gas  to  flow  over  the  cube  c ;  and  though 
the  surface  is  actually  cooled  by  the  passage  of  the  gas,  for 
the  gas  has  to  be  warmed  by  the  metal,  you  see  the  effect 
is  to  augment  considerably  the  radiation :  as  soon  as  the 
gas  begins  to  flow  the  needle  begins  to  move,  and  reaches 
an  amplitude  of  45°. 

We  have  here  varnished  a  metal  by  a  gas,  but  a  more 
interesting  and  subtle  effect  is  the  varnishing  of  one  gaseous 
body  by  another.  I  have  here  a  flask  containing  some  ace- 
tic ether  ;  a  volatile,  and,  as  you  know,  a  highly  absorbent 
substance.  I  attach  the  flask  to  the  experimental  tube,  and 
permit  the  vapour  to  enter  the  tube,  until  the  mercury  col- 
umn has  been  depressed  half  an  inch.  There  is  now  vapour 
possessing  half  an  inch  of  tension  in  the  tube.  I  intend  to 
use  that  vapour  as  my  varnish ;  and  I  intend  to  use  the  ele- 
ment oxygen  instead  of  the  element  gold,  silver,  or  copper, 
as  the  substance  to  which  my  vapour  varnish  is  to  be  ap- 
plied. At  the  present  moment  the  needle  is  at  zero,  and  I 
now  permit  dry  oxygen  to  enter  the  tube :  the  gas  is  dynam- 
ically heated,  and  we  have  seen  its  incompetence  to  radi- 
ate its  heat ;  but  now  it  comes  into  contact  with  the  acetic 
ether  vapour,  and,  communicating  its  motion  to  the  vapour 
by  direct  collision,  the  latter  is  able  to  send  on  the  motion 
to  the  pile.  Observe  the  needle — it  is  caused  to  swing 
through  an  arc  of  70°  by  the  radiation  from  the  vapour 
particles.  I  need  not  insist  upon  the  fact  that  in  this  ex- 
periment the  vapour  bears  precisely  the  same  relation  to 
the  oxygen,  that  the  varnish  does  to  the  metal  in  our  for- 
mer experiments. 

Let  us  wait  a  little,  and  allow  the  vapour  to  pour  away 
the  heat :  it  is  the  discharger  of  the  calorific  force  gene- 


392  LECTURE  XL 

rated  by  the  oxygen — the  needle  is  again  at  zero.  I  work 
the  pump,  the  vapour  within  the  tube  becomes  chilled,  and 
now  you  observe  the  needle  swing  nearly  45°  on  the  other 
side  of  zero.  In  this  way  the  dynamic  radiation  and  ab- 
sorption of  the  vapours  mentioned  in  the  following  table 
have  been  determined;  air,  however,  instead  of  oxygen, 
being  the  substance  employed  to  heat  the  vapour.  The 
limit  of  the  first  swing  of  the  needle  is  noted  as  before. 

Dynamic  Radiation  and  Absorption  of  Vapours. 

Deflections 


Radiation  Absorption 

1.  Bisulphide  of  carbon  14  ...  6 

2.  Iodide  of  methyl       .        .  19'5  ...  8 

3.  Benzol    ....  30  ...  14 

4.  Iodide  of  ethyl  34  15-5 

5.  Methylie  alcohol       .         .  36 

6.  Chloride  of  amyl       ..  41  ...  23 

7.  Amylene         ...  48  ... 

8.  Alcohol            ...  50  ...  27-5 

9.  Sulphuric  ether         .        .  64  ...  34 

10.  Formic  ether  .         .         68  '5     .         .         .38 

11.  Acetic  ether     .        .         .         70        .         .         .     43  , 

We  have  here  used  eleven  different  kinds  of  vapour  as 
varnish  for  our  air,  and  we  find  that  the  dynamic  radiation 
and  absorption  augment  exactly  in  the  order  established  by 
experiments  with  external  sources  of  heat.  We  also  see 
how  beautifully  dynamic  radiation  and  absorption  go  hand 
in  hand,  the  one  augmenting  and  diminishing  with  the 
other. 

The  smallness  of  the  quantity  of  matter  concerned  in 
some  of  these  actions  on  radiant  heat  has  been  often  re- 
ferred to ;  and  I  wish  now  to  describe  an  experiment  which 
shall  furnish  a  more  striking  example  of  this  kind  than  any 
hitherto  brought  before  you.  The  absorption  of  boracic  ether 


VARNISHING   AIR   BY   VAPOUR.  393 

vapour,  as  given  at  page  368,  exceeds  that  of  any  other 
substance  there  referred  to ;  and  its  dynamic  radiation  may 
be  presumed  to  be  commensurate.  I  exhaust  the  experi- 
mental tube  as  perfectly  as  possible,  and  introduce  into  it 
a  quantity  of  boracic  ether  vapour  sufficient  to  depress  the 
mercury  column  -^th  of  an  inch.  The  barometer  stands  to- 
day at  30  inches ;  hence  the  tension  of  the  ether  vapour 
now  in  our  tube  is  -^^-Oih  of  an  atmosphere. 

I  send  dry  air  into  the  tube ;  the  vapour  is  warmed, 
and  the  dynamic  radiation  produces  the  deflection  56°. 

I  work  the  pump  until  I  reduce  the  residue  of  air  within 
it  to  a  tension  of  0'2  of  an  inch,  or  -}£-0-th  of  an  atmosphere. 
A  residue  of  the  boracic  ether  vapour  remains  of  course  in 
the  tube,  the  tension  of  this  residue  being  the  T^sth  part 
of  that  of  the  vapour  when  it  first  entered  the  tube.  I  let 
in  dry  air,  and  find  the  dynamic  radiation  of  the  residual 
vapour  expressed  by  the  deflection  42°. 

I  again  work  the  pump  till  the  tension  of  the  air  within 
it  is  0'2  of  an  inch  ;  the  quantity  of  ether  vapour  now  in 
the  tube  being  il-0ih  of  that  present  in  the  last  experi- 
ment. The  dynamic  radiation  of  this  residue  gives  a  de- 
flection of  20°. 

Two  additional  experiments,  conducted  in  the  same 
way,  gave  deflections  of  14°  and  10°  respectively.  The 
question  now  is,  what  was  the  tension  of  the  boracic  ether 
vapour  when  this  last  deflection  was  obtained  ?  The  fol- 
lowing table  contains  the  answer  to  this  question : — 

Dynamic  Radiation  of  Boracic  Ether. 

Tension  in  parts  of  an  atmosphere  Deflection 

3^0  ...          56 

T6  0  X   3^0    =    4  5  00<)  .  •  42 

lloXlioX  3uo  =  -BTeuooo     •  •  20 

750  X  T60  X  T5U  X  aio  =  TOT26^ OOlOO        •      14 

T«o  x  lio  x  lio  x  Teo  X  airo  =  TSTsTSoOoooo        •  10 

17* 


394:  LECTURE   XI. 

The  air  itself,  warming  the  interior  of  the  tube,  pro- 
duces, as  we  have  seen,  a  deflection  of  7°  ;  hence  the  entire 
deflection  of  10°  was  not  due  to  the  radiation  of  the  va- 
pour. Deducting  7°,  it  would  leave  a  residue  of  3°.  But 
supposing  we  entirely  omit  the  last  experiment,  we  can 
then  have  no  doubt  that  at  least  half  the  deflection  14°  is 
due  to  the  residue  of  boracic  ether  vapour ;  this  residue 
we  find,  by  strict  measurement,  would  have  to  be  multi- 
plied by  one  thousand  millions  to  bring  it  up  to  the  ten- 
sion of  ordinary  atmospheric  air. 

Another  reflection  here  presents  itself,  which  is  worthy 
of  our  consideration.  We  have  measured  the  dynamic 
radiation  of  olefiant  gas,  by  allowing  the  gas  to  enter  our 
tube,  until  the  latter  was  quite  filled.  What  was  the  state 
of  the  warm  radiating  column  of  olefiant  gas  in  this  experi- 
ment ?  It  is  manifest  that  the  portions  of  the  column  most 
distant  from  the  pile  must  radiate  through  the  gas  in  front 
of  them,  and,  in  this  forward  portion  of  the  column  of  gas, 
a  large  quantity  of  the  rays  emitted  by  its  hinder  portion 
will  be  absorbed.  In  fact,  it  is  quite  certain  that  if  we 
made  our  column  sufficiently  long,  the  frontal  portions 
would  act  as  a  perfectly  impenetrable  screen,  to  the  radia- 
tion of  the  hinder  ones.  Thus,  by  cutting  off  the  part  of 
the  gaseous  column  most  distant  from  the  pile,  we  might 
diminish  only  in  a  very  small  degree  the  amount  of  radia- 
tion which  reaches  the  pile. 

Let  us  now  compare  the  dynamic  radiation  of  a  vapour 
writh  that  of  olefiant  gas.  In  the  case  of  vapour  we  use 
only  0*5  of  an  inch  of  tension,  hence  the  radiating  mole- 
cules of  the  ether  are  much  wider  apart  than  those  of  the 
olefiant  gas,  which  have  60  times  the  tension ;  and  conse- 
quently the  radiation  of  the  hinder  portions  of  the  column 
of  vapour  will  have  a  comparatively  open  door  through 
which  to  reach  the  pile.  These  considerations  render  it 
manifest  that  in  the  case  of  the  vapour  a  greater  length  of 


EFFECT   OF   DISTANCE  BETWEEN   RADIANT   CENTRES.    395 

tube  is  available  for  radiation  than  in  the  case  of  olefiant 
gas.  This  leads  to  the  conclusion,  that  if  we  shorten  the 
tube,  we  shall  diminish  the  radiation  in  the  case  of  the 
vapour  more  considerably  than  in  the  case  of  the  gas.  Let 
us  now  bring  our  reasoning  to  the  test  of  experiment. 

We  found  the  dynamic  radiation  of  the  following  four 
substances,  when  the  radiating  column  was  2  feet  9  inches 
long,  to  be  represented  by  the  annexed  deflections  :  — 


Olefiant  gas    .  .  .  .63 

Sulphuric  ether  .  .  .64 

Formic  ether  .  .  .  68  '5 

Acetic  ether    .  .  .  .70 

olefiant  gas  giving  here  the  least  dynamic  radiation. 

Experiments  made  in  precisely  the  same  manner  with  a 
tube  3  inches  long,  or  Jyth  of  the  former  length,  gave  the 
following  deflections  :  — 

Olefiant  gas     .  .  .  .39 

Sulphuric  ether  .  .  .11 

Formic  ether  .  .  .  .12 

Acetic  ether    .  .  .  .15 

The  verification  of  our  reasoning  is  therefore  complete.  It 
is  proved,  that  in  the  long  tube  the  dynamic  radiation  of 
the  vapour  exceeds  that  of  the  gas,  while  in  a  short  one 
the  dynamic  radiation  of  the  gas  exceeds  that  of  the  vapour. 
The  result  proves,  if  proof  were  needed,  that  though  dif- 
fused in  air,  the  vapour  molecules  are  really  the  centres  of 
the  radiation. 

Up  to  the  present  point,  I  have  purposely  omitted  all 
reference  to  the  most  important  vapour  of  all,  as  far  as  our 
world  is  concerned  —  I  mean,  of  course,  the  vapour  of  wa- 
ter. This  vapour,  as  you  know,  is  always  diffused  through 
the  atmosphere.  The  clearest  day  is  not  exempt  from  it  : 


396  LECTURE  XI. 

indeed,  in  the  Alps,  the  purest  skies  are  often  the  most 
treacherous,  the  blue  deepening  with  the  amount  of  aque- 
ous vapour  in  the  air.  It  is  needless,  therefore,  to  remind 
you,  that  when  I  speak  of  aqueous  vapour,  I  mean  nothing 
visible  ;  it  is  not  fog ;  it  is  not  cloud ;  it  is  not  mist  of  any 
kind.  These  are  formed  of  vapour  which  has  been  con- 
densed to  water ;  but  the  blue  vapour  with  which  we  have 
to  deal  is  an  impalpable  transparent  gas.  It  is  diffused 
everywhere  throughout  the  atmosphere,  though  in  very 
different  proportions. 

To  prove  the  existence  of  aqueous  vapour  in  the  air  of 
this  room,  I  have  placed  in  front  of  the  table  a  copper  ves- 
sel, which  was  filled  an  hour  ago  with  a  mixture  of  pound- 
ed ice  and  salt.  The  surface  of  the  vessel  was  then  black, 
but  it  is  now  white — furred  all  over  with  hoar-frost — pro- 
duced by  the  condensation,  and  subsequent  congelation  upon 
its  surface  of  the  aqueous  vapour.  I  can  scrape  off  this 
white  substance,  and  collect  it  in  my  hand.  As  I  remove 
the  frozen  vapour,  the  black  surface  of  the  vessel  reap- 
pears ;  a*nd  now  I  have  collected  a  sufficient  quantity  to 
form  a  respectable  snow-ball.  Let  us  go  one  step  further. 
I  place  this  snow  in  a  mould,  and  squeeze  it  before  you 
into  a  cup  of  ice — there  is  the  cup  ;  and  thus,  without  quit- 
ting this  room,  we  have  experimentally  illustrated  the  man- 
ufacture of  glaciers  from  beginning  to  end.  On  the  plate 
of  glass  which  I  have  used  to  cover  the  vessel  the  vapour 
is  not  congealed,  but  it  is  condensed  so  copiously,  that 
when  I  hold  the  plate  edgeways  the  water  runs  off  it  in  a 
stream. 

The  quantity  of  this  vapour  is  small.  Oxygen  and  ni- 
trogen constitute  about  99£  per  cent,  of  our  atmosphere ; 
of  the  remaining  0'5,  about  0*45  is  aqueous  vapour ;  the  re- 
sidue is  carbonic  acid.  Had  we  not  been  already  acquaint- 
ed with  the  action  of  almost  infinitesimal  quantities  of  mat- 
ter on  radiant  heat,  we  might  well  despair  of  being  able  to 


ABSORPTION   OF   RADIANT   HEAT  BY   HUMID   ALK.       397 

establish  a  measurable  action  on  the  part  of  the  aqueous 
vapour  of  our  atmosphere.  Indeed,  I  quite  neglected  the 
action  of  this  substance  for  a  time,  and  could  hardly  credit 
my  first  result,  which  made  the  action  of  the  aqueous 
vapour  of  our  laboratory  fifteen  times  that  of  the  air  in 
which  it  was  diffused.  This,  however,  by  no  means  ex- 
presses the  true  relation  between  aqueous  vapour  and  dry 
air. 

I  will  make  an  experiment  before  you  which  shall  illus- 
trate this.  Here,  you  see,  I  have  resumed  our  first  ar- 
rangement, as  shown  in  Plate  I.,  with  a  brass  tube,  and 
with  two  sources  of  heat  acting  on  the  opposite  faces  of  the 
pile.  I  exhaust  the  experimental  tube,  and  repeat  to-day 
the  experiment  with  dry  air,  which  I  made  at  the  com- 
mencement of  the  last  lecture.  The  needle  does  not  move 
sensibly.  If  close  to  it  you  would,  as  I  have  already 
stated,  observe  a  motion  through  about  one  degree.  Prob- 
ably, could  we  get  our  air  quite  pure,  its  action  would  be 
even  less  than  this.  I  now  pump  out,  and  allow  the  air  of 
this  room  to  enter  the  experimental  cylinder  direct,  with- 
out permitting  it  to  pass  through  the  drying  apparatus. 
The  needle,  you  observe,  moves  as  the  air  enters,  and  the 
final  deflection  is  48°.  The  needle  will  steadily  point  to 
this  figure  as  long  as  the  sources  of  heat  remain  constant, 
and  as  long  as  the  air  continues  in  the  tube.  These  48° 
correspond  to  an  absorption  of  72  ;  that  is  to  say,  the  aque- 
ous vapour  contained  in  the  atmosphere  of  this  room  to-day 
exerts  an  action  on  the  radiant  heat,  72  times  more  power- 
ful than  that  of  the  air  itself. 

This  result  is  obtained  with  perfect  ease,  still  not  with- 
out due  care.  In  comparing  dry  with  humid  air  it  is  per- 
fectly essential  that  the  substances  be  pure.  You  may  work 
for  months  with  an  imperfect  drying  apparatus  and  fail  to 
obtain  air,  which  shows  this  almost  total  absence  of  action 
on  radiant  heat.  An  amount  of  organic  impurity,  too  small 


398  LECTURE   XI. 

to  be  seen  by  the  eye,  is  sufficient  to  augment  fiftyfold  the 
action  of  the  ah*.  Knowing  the  effect  which  an  almost  in- 
finitesimal amount  of  matter,  in  certain  cases,  can  produce, 
you  are  better  prepared  for  such  facts  than  I  was  when  they 
first  forced  themselves  upon  my  attention.  But  let  us  be 
careful  in  our  enquiries.  The  experimental  result  which 
we  have  just  obtained  will,  if  true,  have  so  important  an 
influence  on  the  science  of  meteorology,  that,  before  it  is 
admitted,  it  ought  to  be  subjected  to  the  closest  scrutiny. 
First  of  all,  look  at  this  piece  of  rocksalt  brought  in  from 
the  next  room,  where  it  has  stood  for  some  time  near  a 
tank,  but  not  in  contact  with  visible  moisture.  The  salt  is 
wet ;  it  is  a  hygroscopic  substance,  and  freely  condenses 
moisture  upon  its  surface.  Here,  also,  is  a  polished  plate 
of  the  substance,  which  is  now  quite  dry ;  I  breathe  upon 
it,  and  instantly  its  affinity  for  moisture  causes  the  vapour 
of  my  breath  to  overspread  the  surface  in  a  film  which  ex- 
hibits beautifully  the  colours  of  thin  plates.*  Now  we 
know  from  the  table,  at  page  313,  how  opaque  a  solution 
of  rocksalt  is  to  the  calorific  rays,  and  hence  arises  the 
question  whether,  in  the  above  experiment  with  undried 
air,  we  may  not  in  reality  be  measuring  the  action  of  a  thin 
stratum  of  such  a  solution,  deposited  on  our  plates  of  salt, 
instead  of  the  pure  action  of  the  aqueous  vapour  of  the  air. 
If  you  operate  incautiously,  and,  more  particularly,  if  it 
be  your  actual  intention  to  wet  your  plates  of  salt,  you  may 
readily  obtain  the  deposition  of  moisture.  This  is  a  point 
on  which  any  competent  experimenter  will  soon  instruct 
himself;  but  the  essence  of  good  experimenting  consists  in 
the  exclusion  of  circumstances  which  would  render  the  pure 
and  simple  questions  which  we  intend  to  put  to  Nature, 
impure  and  composite  ones.  The  first  way  of  replying  to 
the  doubt  here  raised  is  to  examine  our  plates  of  salt ;  if 
the  experiments  have  been  properly  conducted,  no  trace  of 
moisture  is  found  upon  the  surface.  To  render  the  success 
*  See  Note  (8)  at  the  erd  of  this  Lecture. 


WETTING  PwOCKSALT  PLATES. 


399 


of  this  experiment  more  certain,  I  will  slightly  alter  the  ar- 
rangement of  our  apparatus.  Hitherto  we  have  had  the 
thermo-electric  pile  and  its  two  reflectors  entirely  outside 
the  experimental  cylinder.  I  now  take  this  reflector  from 
the  pile,  and  removing  this  terminal  plate  of  rocksalt,  I 
push  the  reflector  into  the  cylinder.  The  hollow  reflecting 
cone  is  '  sprung '  at  its  base  a  b  (fig.  95),  (our  former  ar- 
rangement, with  the  single  exception  that  one  of  the  re- 
flectors of  the  pile  P  is  now  within  the  tube)  so  that  it  is 

Fig.  95. 


held  tightly  by  its  own  pressure  against  the  inner  surface 
of  the  cylinder.  The  space  between  the  outer  surface  of 
the  reflector  and  the  inner  surface  of  the  tube  I  fill  with 
fragments  of  fused  chloride  of  calcium,  which  are  prevent- 
ed from  falling  out  by  a  little  screen  of  wire  gauze.  I  now 
reattach  my  plate  of  salt,  against  the  inner  surface  of  which 
abuts  the  narrow  end .  of  the  reflector ;  bring  the  face  of 
the  pile  close  up  to  the  plate,  though  not  into  actual  con- 
tact with  it,  and  now  our  arrangement  is  complete. 

In  the  first  place  it  is  to  be  remarked,  that  the  plate  of 
salt  nearest  to  the  source  of  heat  c  is  never  moistened,  un- 
less the  experiments  are  of  the  grossest  character.  Its 
proximity  to  the  source  makes  it  the  track  of  a  flux  of  heat, 
powerful  enough  to  chase  away  every  trace  of  humidity 


4:00  LECTURE   XL 

from  its  surface.  The  distant  plate  is  the  one  in  danger, 
and  now  we  have  the  circumferential  portions  of  this  plate 
kept  perfectly  dry  by  the  chloride  of  calcium  ;  no  moist  air 
can  at  all  reach  the  rim  of  the  plate  ;  while  upon  its  central 
portion,  measuring  about  a  square  inch  in  area,  we  have 
converged  our  entire  radiation.  On  d  priori  grounds  we 
should  conclude  that  it  is  quite  impossible  that  a  film  of 
moisture  could  collect  there ;  and  this  conclusion  is  justified 
by  fact.  I  test,  as  before,  the  dried  air  and  the  undried 
air  of  this  room,  and  find,  as  in  the  former  instance,  that 
the  latter  produces  seventy  times  the  effect  of  the  former. 
The  needle  is  now  deflected  by  the  absorption  of  the  un- 
dried air ;  allowing  this  air  to  remain  in  the  tube,  I  unscrew 
my  plate  of  salt,  and  examine  its  surface.  I  even  use  a 
lens  for  this  purpose,  taking  care,  however,  that  my  breath 
does  not  strike  the  plate.  It  was  carefully  polished  when 
attached  to  the  tube  ;  it  is  perfectly  polished  now.  Glass, 
or  rockcrystal,  could  not  show  a  surface  more  exempt  from 
any  appearance  of  moisture.  I  place  a  dry  handkerchief 
over  my  finger,  and  draw  it  along  the  surface :  it  leaves  no 
trace  behind.  There  is  not  the  slightest  deposition  of  mois- 
ture ;  still  we  see  that  absorption  has  taken  place.  This 
experiment  is  conclusive  against  the  hypothesis  that  the 
effects  observed  are  due  to  a  film  of  brine  instead  of  to 
aqueous  vapour. 

The  doubt  may,  however,  linger,  that  although  we  are 
unable  to  detect  the  film  of  moisture,  it  may  still  be  there. 
This  doubt  is  answered  in  the  following  way : — I  detach 
the  experimental  tube  from  the  front  chamber,  and  remove 
the  two  plates  of  rocksalt ;  the  tube  is  now  open  at  both 
ends,  and  my  aim  will  be  to  introduce  dry  and  moist  air 
into  this  open  tube,  and  to  compare  their  effects  upon  the 
radiation  from  our  source.  And  here,  as  in  all  other  cases, 
the  practical  tact  of  the  experimenter  must  come  into  play. 
The  source,  on  the  one  hand,  and  the  pile  on  the  other,  are 


. 

ABSOKPTION  WITHOUT  EOCKSALT  PLATES.1   .  401 

N^s.*Q.  /' . 

o^ 

now  freely  exposed  to  the  air ;  and  a  very  slight  agitation 
acting  upon  either  would  disturb,  and  might,  indeed,  alto- 
gether mask  the  effect  we  seek.  The  air,  then,  must  be  in- 
troduced into  the  open  tube,  without  producing  any  com- 
motion either  near  the  source  or  near  the  pile.  The  length 
of  the  experimental  tube  is  now  4  feet  3  inches ;  at  c  (fig. 
96)  is  a  cock  connected  with  an  India-rubber  bag  containing 
common  air,  and  subjected  by  a  weight  to  gentle  pressure  ; 

Fig.  96. 


at  D  is  a  second  cock  connected  by  a  flexible  tube,  £,  with 
an  air-pump ;  between  the  cock  c  and  the  India-rubber  bag 
our  drying  tubes  are  introduced ;  when  a  cock  near  the  bag 
is  opened,  the  air  is  forced  gently  through  the  drying  tubes 
into  the  experimental  cylinder.  The  air-pump  is  slowly 
worked  at  the  same  time,  and  the  dry  air  thereby  drawn 
towards  D.  The  distance  of  c  from  the  source  s  is  18 
inches,  and  the  distance  of  D  from  the  pile  p  is  12  inches, 
the  compensating  cube  c,  and  the  screen  H,  serve  the  same 
purpose  as  before.  By  thus  isolating  the  central  portion 
of  the  tube,  we  can  displace  dry  air  by  moist,  or  moist  air 
by  dry,  without  permitting  any  agitation  to  reach  either 
the  source  or  the  pile. 

At  present  the  tube  is  filled  with  the  common  air  of  the 
laboratory,  and  the  needle  of  the  galvanometer  points 
steadily  to  zero.  I  now  allow  air  to  pass  through  the  dry- 
ing apparatus  and  to  enter  the  open  tube  at  c,  the  pump 
being  worked  at  the  same  time.  Mark  the  effect.  When 
the  dry  air  enters  the  needle  commences  to  move,  and  the 


4:02  LECTURE   XI. 

direction  of  its  motion  shows  that  more  heat  is  now  pass- 
ing than  before.  The  substitution  of  dry  air  for  the  air  of 
the  laboratory  has  rendered  the  tube  more  transparent  to 
the  rays  of  heat.  The  final  deflection  thus  obtained  is  45 
degrees.  Here  the  needle  steadily  remains,  and  beyond 
this  point  it  cannot  be  moved  by  any  further  pumping  in 
of  the  dry  air. 

I  now  shut  off  the  supply  of  dry  air  and  cease  working 
the  pump  ;  the  needle  sinks,  but  with  great  slowness,  indi- 
cating a  correspondingly  slow  diffusion  of  the  aqueous 
vapour  of  the  adjacent  air  into  the  dry  air  of  the  tube.  If 
I  work  the  pump  I  hasten  the  removal  of  the  dry  air,  and 
the  needle  sinks  more  speedily, — it  now  points  to  zero. 
The  experiment  may  be  made  a  hundred  times  in  succes- 
sion without  any  deviation  from  this  result ;  on  the  en- 
trance of  the  dry  air  the  needle  invariably  goes  up  to  45°, 
showing  the  augmented  transparency ;  on  the  entrance  of 
the  undried  air  the  needle  sinks  to  0°,  showing  augmented 
absorption. 

But  the  atmosphere  to-day  is  not  saturated  with  mois- 
ture ;  hence,  if  I  saturate  the  air,  I  may  expect  to  get  a 
greater  action.  I  remove  the  drying  apparatus  and  put  in 
its  place  a  U  tube,  which  is  filled  with  fragments  of  glass 
moistened  by  distilled  water.  Through  this  tube  I  force 
the  air  from  the  India-rubber  bag,  and  work  the  pump  as 
before.  We  are  now  displacing  the  humid  air  of  the  labor- 
atory by  still  more  humid  air,  and  see  the  consequence. 
The  needle  moves  in  a  direction  which  indicates  augmented 
opacity,  the  final  deflection  being  15°. 

Here  then  we  have  substantially  the  same  result  as  that 
obtained  when  we  stopped  our  tube  with  plates  of  rock- 
salt  ;  hence  the  action  cannot  be  referred  to  a  hypothetical 
film  of  moisture  deposited  upon  the  surface  of  the  plates. 
And  be  it  remarked  that  there  is  not  the  slightest  caprice 
or  uncertainty  in  these  experiments  when  properly  con- 


PROPORTION   ABSORBED  BY   HUMID  AIR.  403 

ducted.  They  have  been  executed  at  different  times  and 
seasons ;  the  tube  has  been  dismounted  and  remounted  ; 
the  suggestions  of  eminent  men  who  have  seen  the  experi- 
ments, and  whose  object  it  was  to  test  the  results,  have 
been  complied  with  ;  but  no  deviation  from  the  effects  just 
recorded  has  been  observed.  The  entrance  of  each  kind 
of  air  is  invariably  accompanied  by  its  characteristic  action ; 
the  needle  is  under  the  most  complete  control :  in  short,  no 
experiments  hitherto  made  with  solid  and  liquid  bodies,  are 
more  certain  in  their  execution,  than  the  foregoing  experi- 
ments on  dry  and  humid  air. 

We  can  easily  estimate  the  per  centage  of  the  entire 
radiation  absorbed  by  the  common  air  between  the  points 
c  and  D. 

Introducing  this  tin  screen  between  the  experimental 
cylinder  and  the  pile,  I  shut  off  one  of  the  sources  of  heat. 
The  deflection  produced  by  the  other  source  indicates  the 
total  radiation. 

This  deflection  corresponds  to  about  780  of  the  units 
which  have  been  hitherto  adopted;  one  unit  being  the 
quantity  of  heat  necessary  to  move  the  needle  from  0°  to 
1°.  The  deflection  of  45°  corresponds  to  62  units ;  out  of 
780,  therefore,  62,  in  this  instance,  have  been  absorbed  by 
the  moist  air.  The  following  statement  gives  us  the  ab- 
sorption per  hundred : — 

780: 100  =  62:  7-9. 

An  absorption  of  nearly  8  per  cent,  was,  therefore,  ef- 
fected by  the  atmospheric  vapour  which  occupied  the  tube 
between  c  and  D.  Air  perfectly  saturated  gives  a  still 
greater  absorption. 

This  absorption  took  place,  notwithstanding  the  partial 
sifting  of  the  heat,  in  its  passage  from  the  source  to  c,  and 
from  D  to  the  pile.  The  moist  air,  moreover,  was,  prob- 
ably, only  in  part  displaced  bv  the  dry.  In  other  experi- 


404:  LECTURE   XI. 

ments  I  found,  with  a  tube  4  feet  long,  and  polished  within, 
that  the  atmdspheric  vapour,  on  a  day  of  average  dryness, 
absorbed  over  6  per  cent,  of  the  radiation  from  our  source. 
Regarding  the  earth  as  a  source  of  heat,  no  doubt,  at  least 
10  per  cent  of  its  heat  is  intercepted  within  ten  feet  of  the 
surface.*  This  single  fact  suggests  the  enormous  influence 
which  this  newly  developed  property  of  aqueous  vapour 
must  have  in  the  phenomena  of  meteorology. 

But  we  have  not  yet  disposed  of  all  objections.  It  has 
been  intimated  to  me  that  the  air  of  our  laboratory  might 
be  impure  ;  and  the  suspended  carbon  particles  of  the  Lon- 
don air  have  also  been  referred  to,  as  a  possible  cause  of  the 
absorption,  ascribed  to  aqueous  vapour. 

I  reply :  1st.  The  results  were  obtained  when  the  ap- 
paratus was  removed  from  the  laboratory — they  are  ob- 
tainable in  this  room.  2ndly.  Air  was  brought  from  the 
following  localities  in  impervious  bags : — Hyde  Park,  Prim- 
rose Hill,  Hampstead  Heath,  Epsom  Downs  (near  the 
Grand  Stand)  ;  a  field  near  Newport,  Isle  of  Wight ;  St. 
Catharine's  Down,  Isle  of  Wight;  the  sea  beach  near 
Black-gang  Chine.  The  aqueous  vapour  of  the  air  from 
all  these  localities,  examined  in  the  ^lsual  way,  exerted  an 
absorption  seventy  times  that  of  the  air  in  which  the  vapour 
was  diffused. 

Again,  I  experimented  thus.  The  air  of  the  laboratory 
was  dried  and  purified  until  its  absorption  fell  below  unity ; 
this  purified  air  was  then  led  through  a  U  tube,  filled  with 
fragments  of  perfectly  clean  glass  moistened  with  distilled 
water.  Its  neutrality,  when  dry,  showed  that  all  prejudi- 
cial substances  had  been  removed  from  it,  and  in  passing 
through  the  U  tube,  it  could  take  up  nothing  but  the  pure 
vapour  of  water.  The  vapour  thus  carried  into  the  experi- 

*  Under  some  circumstances  the  absorption,  I  have  reason  to  believe, 
considerably  exceeds  this  amount. 


OBJECTIONS   ANSWERED.  405 

mental  tube  produced  an  action  ninety  times  greater  than 
that  of  the  air  which  carried  it. 

But  fair  and  philosophic  criticism  does  not  end  even 
here.  The  tube  with  which  these  experiments  were  made 
is  polished  within,  and  it  was  surmised  that  the  vapour  of 
the  humid  air  might,  on  entering,  have  deposited  itself 
upon  the  interior  surface  of  the  tube,  thus  diminishing  its 
reflective  power,  and  producing  an  effect  apparently  the 
same  as  absorption.  But  why,  I  would  ask,  should  such  a 
deposition  of  moisture  take  place  ?  On  many  of  the  days 
when  these  experiments  were  made  the  air  was  at  least  25 
per  cent,  under  its  point  of  saturation.  It  can  hardly  be 
assumed  that  such  air  would  deposit  its  moisture  on  a  me- 
tallic surface,  against  which,  moreover,  the  rays  from  our 
source  of  heat  were  at  the  time  impinging.  The  mere  con- 
sideration of  the  objection  must  deprive  it  of  weight. 
Further,  the  absorption  is  exerted  when  only  a  small  frac- 
tion of  an  atmosphere  is  introduced  into  the  tube,  and  it  is 
proportional  to  the  quantity  of  air  present.  This  is  shown 
by  the  following  table,  which  gives  the  absorption,  by  hu- 
mid air,  at  tensions  varying  from  5  to  30  inches  of  mercury. 

Humid  Air. 

Absorption 

Tension  , • , 

in  inches  Observed  Calculated 

5 16    ...   16 

10 32    ...  32 

15 49    ...  48 

20 64    .    .    •  64 

25 82    ...  80 

30 98    ...  96 

The  third  column  of  this  table  is  calculated  on  the  as- 
sumption that  the  absorption  is  proportional  to  the  quan- 
tity of  vapour  in  the  tube,  and  the  agreement  of  the  calcu- 
lated and  observed  results  show  this  to  be  the  case,  within 
the  limits  of  the  experiment.  It  cannot  be  supposed  that 


406 


LECTURE   XI. 


effects  so  regular  as  these,  and  agreeing  so  completely  with 
those  obtained  with  small  quantities  of  other  vapours,  and 
even  with  small  quantities  of  the  permanent  gases,  can  be 
due  to  the  condensation  of  the  vapour  on  the  interior  sur- 
face. When,  moreover,  five  inches  of  air  were  in  the  tube, 
less  than  J  th  of  the  vapour  necessary  to  saturate  the  space 
was  present.  The  dryest  day  would  make  no  approach  to 
this  dryness.  Condensation  under  these  circumstances  is 
impossible,  and  more  especially  a  condensation  which 
should  destroy,  by  its  action  upon  the  inner  reflector,  quan- 
tities of  heat  so  accurately  proportional  to  the  quantities  of 
matter  present. 

My  desire,  however,  was  to  take  this  important  ques- 
tion quite  out  of  the  domain  of  mere  reasoning,  however 
strong  this  might  appear.  I  therefore  resolved  to  abandon 
not  only  the  plates  of  rocksalt  but  also  the  experimental 
tube  itself,  and  to  displace  one  portion  of  the  free  atmos- 

Fig.  97. 


phere  by  another.  With  this  view  the  following  arrange- 
ment was  made : — c  (fig.  97),  a  cube  of  boiling  water,  is 
our  source  of  heat.  Y  is  a  hollow  brass  cylinder  set  up- 
right, 3-5  inches  wide,  and  7*5  inches  high,  p  is  the  ther- 


ABSORPTION   OF   AQUEOUS   VAPOUK  IN   FREE   AIR.    407 

mo-electric  pile,  and  c'  a  compensating  cube,  between  which 
and  P  is  an  adjusting  screen,  to  regulate  the  amount  of  ra- 
diation falling  on  the  posterior  surface  of  the  pile.  The 
whole  arrangement  was  surrounded  by  a  hoarding,  the 
space  within  which  was  divided  into  compartments  by 
sheets  of  tin,  and  these  spaces  were  stuffed  loosely  with 
paper  or  horsehair.  These  precautions,  which  required 
time  to  be  learned,  were  necessary  to  prevent  the  formation 
of  local  air-currents,  and  also  to  intercept  the  irregular  ac- 
tion of  the  external  air.  The  effect  to  be  measured  here  is 
very  small,  and  hence  the  necessity  of  removing  all  causes 
of  disturbance  which  could  possibly  interfere  with  its  clear- 
ness and  purity. 

A  rose-burner  r  was  placed  at  the  bottom  of  the  cylin- 
der Y,  and  from  it  a  tube  passed  to  an  India-rubber  bag 
containing  air.  The  cylinder  Y  was  first  filled  with  frag- 
ments of  rockcrystal,  moistened  with  distilled  water.  On 
subjecting  the  India-rubber  bag  to  pressure,  the  air  from  it 
was  gently  forced  up  among  the  fragments  of  quartz,  and 
having  there  charged  itself  with  vapour  it  was  discharged 
in  the  space  between  the  cube  c  and  the  pile.  Previous  to 
this  the  needle  stood  at  zero  ;  but  on  the  emergence  of  the 
saturated  air  from  the  cylinder,  the  needle  moved  and  took 
up  a  final  deflection  of  five  degrees.  The  direction  of  the 
deflection  showed  that  the  opacity  of  the  space  between 
the  source  c  and  the  pile  was  augmented  by  the  presence 
of  the  saturated  air. 

The  quartz  fragments  were  now  removed,  and  the  cyl- 
inder was  filled  with  fragments  of  fresh  chloride  of  cal- 
cium, through  which  the  air  was  gently  forced,  exactly  as 
in  the  last  experiment.  Now,  however,  in  passing  through 
the  chloride  of  calcium,  it  was  in  great  part  robbed  of  its 
aqueous  vapour,  and  the  air,  thus  dried,  displaced  the  com- 
mon air  between  the  source  and  pile.  The  needle  moved, 
declaring  a  permanent  deflection  of  10  degrees;  the  direc- 


408  LECTUKE   XI. 

tion  of  the  deflection  showed  that  the  transparency  of  the 
space  was  augmented  by  the  presence  of  the  dry  air.  By 
properly  timing  the  discharges  of  the  air,  the  swing  of  the 
needle  could  be  augmented  to  15  or  20  degrees.  Repeti- 
tion showed  no  deviation  from  this  result ;  the  saturated 
air  always  augmented  the  opacity,  the  dry  air  always  aug- 
mented the  transparency  of  the  space  between  the  source 
and  the  pile.  Not  only,  therefore,  have  the  plates  of  rock- 
salt  been  abandoned,  but  also  the  experimental  tube  itself, 
and  the  results  are  all  perfectly  concurrent  as  regards  the 
action  of  aqueous  vapour  upon  radiant  heat. 

Were  this  subject  less  important  I  should  not  have 
dwelt  upon  it  so  long.  I  thought  it  right  to  remove  every 
objection,  so  that  meteorologists  might  apply,  without  the 
faintest  misgiving,  the  results  of  experiments.  The  appli- 
cations of  these  results  to  their  science  must  be  innumer- 
able ;  and  here  I  cannot  but  regret  that  the  incompleteness 
of  my  knowledge  prevents  me  from  making  the  proper  ap- 
plications myself.  I  would,  however,  ask  your  permission  to 
refer  to  such  points  as  I  can  now  call  to  mind,  with  which 
the  facts  just  established  appear  to  be  more  or  less  inti- 
mately connected. 

And,  first,  it  is  to  be  remarked  that  the  vapour  which 
absorbs  heat  thus  greedily,  radiates  it  copiously.  This 
fact  must,  I  imagine,  come  powerfully  into  play  in  the 
tropics.  We  know  that  the  sun  raises  from  the  equatorial 
ocean  enormous  quantities  of  vapour,  and  that  immediately 
under  him,  in  the  region  of  calms,  the  rain,  due  to  the  con- 
densation of  the  vapour,  descends  in  deluges.  Hitherto, 
this  has  been  ascribed  to  the  chilling  which  accompanies 
the  expansion  of  the  ascending  air,  and  no  doubt  this,  as  a 
true  cause,  must  produce  its  proportional  effect.  But  I 
cannot  help  thinking  that  the  radiation  from  the  vapour  it- 
self is  also  influential.  Imagine  a  column  of  saturated  air 
ascending  from  the  equatorial  ocean ;  for  a  time  the  vapour 


TOEKENTIAL   RAINS   OF   THE  TEOPICS.  409 

entangled  in  this  air,  is  surrounded  by  air  almost  fully 
saturated.  Its  vapour  radiates,  but  it  radiates  into  vapour, 
and  the  vapour  into  it.  To  the  radiation  from  any  vapour, 
a  screen  of  the  same  vapour  is  particularly  opaque.  Hence, 
for  a  time,  the  radiation  from  our  ascending  column  is  in- 
tercepted, and  in  great  part  returned  by  the  surrounding 
vapour  ;  condensation  under  such  circumstances  cannot  oc- 
cur. But  the  quantity  of  aqueous  vapour  in  the  air  dimin- 
ishes speedily  as  we  ascend ;  the  decrement  of  tension,  as 
proved  by  the  observations  of  Hooker,  Strachy,  and  Welsh, 
is  much  more  speedy  than  that  of  the  air ;  and,  finally,  our 
vaporous  column  finds  itself  elevated  beyond  the  protecting 
screen  which,  during  the  first  portion  of  its  ascent,  was 
spread  out  above  it.  It  is  now  in  the  presence  of  pure  space, 
and  into  space  it  pours  its  heat  without  stoppage  or  re- 
quital. To  the  loss  of  heat  thus  endured,  the  condensation 
of  the  vapour,  and  its  torrential  descent  to  the  earth,  must 
certainly  be  in  part  ascribed. 

Similar  remarks  apply  to  the  formation  of  cumuli  in  our 
own  latitudes  ;  they  are  the  heads  of  columnar  bodies  of 
vapour  which  rise  from  the  earth's  surface,  and  are  precipi- 
tated as  soon  as  they  reach  a  certain  elevation.  Thus  the 
visible  cloud  forms  the  capital  of  an  invisible  pillar  of  sat- 
urated air.  Certainly  the  top  of  such  a  column,  raised 
above  the  vapour  screen  which  clasps  the  earth,  and  offer- 
ing itself  to  space  must  be  chilled  by  radiation  ;  in  this  ac- 
tion alone  we  have  a  physical  cause  for  the  generation  of 
clouds. 

Mountains  act  as  condensers,  but  how?  Partly,  no 
doubt,  by  the  coldness  of  their  own  masses ;  which  cold- 
ness they  owe  to  their  elevation.  Above  them  spreads  no 
vapour  screen  of  sufficient  density  to  intercept  their  heat, 
which  consequently  gushes  unrequited  into  space.  When 
the  sun  is  withdrawn,  this  loss  is  shown  by  the  quick  and 
large  descent  of  the  thermometer.  This  descent  is  not  due 
18 


410  LECTURE    XI. 

to  radiation  from  the  air,  but  to  radiation  from  the  earth, 
or  from  the  thermometer  itself.  Thus  the  difference  be- 
tween a  thermometer  which,  properly  confined,  gives  the 
true  temperature  of  the  night  air,  and  one  which  is  permit- 
ted to  radiate  freely  towards  space,  must  be  greater  at  high 
elevations  than  at  low  ones.  This  conclusion  is  entirely 
confirmed  by  observation.  On  the  Grand  Plateau  of  Mont 
Blanc,  for  example,  MM.  Martins  and  Bravais  found  the 
difference  between  two  such  thermometers  to  be  24°  Fahr. ; 
when  a  difference  of  only  10°  was  observed  at  Chamouni. 

But  mountains  also  act  as  condensers  by  the  deflection 
upwards  of  moist  winds,  and  their  consequent  expansion ; 
the  chilling  thus  produced  is  the  same  as  that  which  accom- 
panies the  direct  ascent  of  a  column  of  warm  air  into  the 
atmosphere  ;  the  elevated  air  performs  work,  and  its  heat 
is  correspondingly  consumed.  But  in  addition  to  these 
causes,  I  think  we  must  take  into  account  the  radiant 
power  of  the  moist  air  when  thus  tilted  upwards.  It  is 
thereby  lifted  beyond  the  protection  of  the  aqueous  layer 
which  lies  close  to  the  earth,  and  therefore  pours  its  heat 
freely  into  space,  thus  effecting  its  own  condensation.  No 
doubt,  I  think,  can  be  entertained,  that  the  extraordinary 
energy  of  water  as  a  radiant,  in  all  its  states  of  aggrega- 
tion, must  play  a  powerful  part  in  the  condensation  of  a 
mountain  region.  As  vapour  it  pours  its  heat  into  space 
and  promotes  condensation ;  as  liquid  it  pours  its  heat  into 
space  and  promotes  congelation ;  as  snow  it  pours  its  heat 
into  space  and  thus  converts  the  surfaces  on  which  it  falls 
into  more  powerful  condensers  than  they  otherwise  would 
be.  Of  the  numerous  wonderful  properties  of  water,  not 
the  least  important  is  this  extraordinary  power  which  it 
possesses,  of  discharging  the  motion  of  heat  upon  the  in- 
terstellar ether. 

A  freedom  of  escape  similar  to  that  from  bodies  of 
vapour  at  great  elevations  would  occur  at  the  earth's  sur- 


COLD  OF  CENTRAL  ASIA,   ETC.  411 

face  generally,  were  the  aqueous  vapour  removed  from  the 
air  above  it,  for  the  body  of  the  atmosphere  in  a  practical 
vacuum  as  regards  the  transmission  of  radiant  heat.  The 
withdrawal  of  the  sun  from  any  region  over  which  the  at- 
mosphere is  dry  must  be  followed  by  quick  refrigeration. 
The  moon  would  be  rendered  entirely  uninhabitable  by  be- 
ings like  ourselves  through  the  operation  of  this  single 
cause ;  with  an  outward  radiation  uninterrupted  by  aqueous 
vapour,  the  difference  between  her  monthly  maxima  and 
minima  must  be  enormous.  The  winters  of  Thibet  are  al- 
most unendurable  from  the  same  cause.  Witness  how  the 
isothermal  lines  dip  from  the  north  into  Asia,  in  winter,  as 
a  proof  of  the  low  temperature  of  this  region.  Humboldt 
has  dwelt  upon  the  '  frigorific  power  '  of  the  central  por- 
tions of  this  continent,  and  controverted  the  idea  that  it 
was  to  be  explained  by  reference  to  its  elevation,  for  there 
were  vast  expanses  of  country,  not  much  above  the  sea 
level,  with  an  exceedingly  low  temperature.  But  not  know- 
ing the  influence  which  we  are  now  studying,  Humboldt,  I 
imagine,  omitted  one  of  the  most  important  of  the  causes 
which  contributed  to  the  observed  result.  Even  the  ab- 
sence of  the  sun  at  night  causes  powerful  refrigeration 
when  the  air  is  dry.  The  removal,  for  a  single  summer 
night,  of  the  aqueous  vapour  from  the  atmosphere  which 
covers  England,  would  be  attended  by  the  destruction  of 
every  plant  which  a  freezing  temperature  could  kill.  In 
Sahara,  where  '  the  soil  is  fire  and  the  wind  is  flame,'  the 
refrigeration  at  night  is  often  painful  to  bear.  Ice  has  been 
formed  in  this  region  at.  night.  In  Australia,  also,  the  di- 
urnal range  of  temperature  is  very  great,  amounting,  com- 
monly, to  between  40  and  50  degrees.  In  short,  it  may  be 
safely  predicted,  that  wherever  the  air  is  dry,  the  daily 
thermometric  range  will  be  great.  This,  however,  is  quite 
different  from  saying  that  when  the  air  is  clear  the  ther- 
mometric range  will  be  great.  Great  clearness  to  light  is 


412 


LECTURE   XI. 


perfectly  compatible  with  great  opacity  to  heat ;  the  at- 
mosphere may  be  charged  with  aqueous  vapour  while  a 
deep  blue  sky  is  overhead,  and  on  such  occasions  the  ter- 
restrial radiation  would,  notwithstanding  the  ;  clearness,' 
be  intercepted. 

And  here  we  are  led  to  an  easy  explanation  of  a  fact 
which  evidently  perplexed  Sir  John  Les- 
lie. This  celebrated  experimenter  con- 
structed an  instrument  which  he  named 
an  cethrioscope,  the  function  of  which 
was  to  determine  the  radiation  against 
the  sky.  It  consisted  of  two  glass  bulbs 
united  by  a  vertical  glass  tube,  so  nar- 
row that  a  little  column  of  liquid  was 
supported  in  the  tube  by  its  own  adhe- 
sion. The  lower  bulb  D  (fig.  98)  was 
protected  by  a  metallic  envelope,  and 
gave  the  temperature  of  the  air ;  the  up- 
per bulb  B,  was  blackened,  and  was  sur- 
rounded by  a  metallic  cup  c,  which  pro- 
tected the  bulb  from  terrestrial  radia- 
tion. 

'  This  instrument,'  says  its  inventor, 
6  exposed  to  the  open  air  in  clear  weath- 
er will  at  all  times,  both  during  the  day 
and  the  night,  indicate  an  impression  of 
cold  shot  downwards  from  the  higher 
The  sensibility  of  the  instrument  is  very 
striking,  for  the  liquor  incessantly  falls  and  rises  in  the 
stem,  with  every  passing  cloud.  But  the  cause  of  its  varia- 
tions does  not  always  appear  so  obvious.  Under  a  fine 
blue  sky  the  cethrioscope  will  sometimes  indicate  a  cold  of 
50  millesimal  degrees ;  yet  on  other  days,  when  the  air 
seems  equally  bright,  the  effect  is  hardly  30°.'  This  anom- 
aly is  simply  due  to  the  difference  in  the  quantity  of 


regions. 


'CLEAK'  DAYS  AND  <DKY'  DAYS.      413 

aqueous  vapour  present  in  the  atmosphere.  Indeed,  Leslie 
himself  connects  the  effect  with  aqueous  vapour  in  these 
words,  '  The  pressure  of  hygrometric  moisture  in  the  air 
probably  affects  the  instrument.'  It  is  not,  however,  the 
'  pressure '  that  is  effective ;  the  presence  of  invisible  vapour 
intercepted  the  radiation  from  the  sethrioscope,  while  its 
absence  opened  a  door  for  the  escape  of  this  radiation  into 
space.  As  .regards  experiments  on  terrestrial  radiation,  a 
new  definition  will  have  to  be  given  for  '  a  clear  day ; '  it 
is  manifest,  for  example,  that  in  experiments  with  the 
pyrheliometer,*  two  days  of  equal  visual  clearness  may 
give  totally  different  results.  We  are  also  enabled  to  ac- 
count for  the  fact  that  the  radiation  from  this  instrument 
is  often  intercepted  when  no  cloud  is  seen.  Could  we, 
however,  make  the  constituents  of  the  atmosphere,  its 
vapour  included,  objects  of  vision,  we  should  see  sufficient 
to  account  for  this  result. 

Another  interesting  point  on  which  this  subject  has  a 
bearing  is  Melloni's  theory  of  serein.  '  Most  authors,' 
writes  this  eminent  philosopher,  4  attribute  to  the  cold,  re- 
sulting from  the  radiation  of  the  air,  the  excessively  fine 
rain  which  sometimes  falls  in  a  clear  sky,  during  the  fine 
season,  a  few  moments  after  sunset.'  '  But,'  he  continues, 
'  as  no  fact  is  yet  known  which  directly  proves  the  emissive 
power  of  pure  and  transparent  elastic  fluids,  it  appears  to 
me  more  conformable,'  &c.,  &c.  If  the  difficulty  here 
urged  against  the  theory  of  serein  be  its  only  one,  the  the- 
ory will  stand,  for  transparent  elastic  fluids  are  now  proved 
to  possess  the  power  of  radiation  which  the  theory  assumes. 
It  is  not,  however,  to  radiation  from  the  air  that  the  chil- 
ling can  be  ascribed,  but  to  radiation  from  the  body  itself, 
whose  condensation  produces  the  serein. 

Let  me  add  the  remark,  that  as  far  as  I  can  at  present 

*  The  instrument  is  described  in  Lecture  XII. 


414:  LECTUKE   XT. 

judge,  aqueous  vapour  and  liquid  water  absorb  the  same 
class  of  rays  ;  this  is  another  way  of  stating  that  the  colour 
of  pure  water  is  shared  by  its  vapour.  In  virtue  of  aqueous 
vapour  the  atmosphere  is  therefore  a  blue  medium.  I  be- 
lieve it  has  been  remarked  that  the  colour  of  the  firmament- 
al  blue,  and  of  distant  hills,  deepens  with  the  amount  of 
aqueous  vapour  in  the  air ;  but  the  substance  which  pro- 
duces a  variation  of  depth  must  be  effective  as  an  origin 
of  color.  Whether  the  azure  of  the  sky — the  most  difficult 
question  of  meteorology, — is  to  be  thus  accounted  for,  I 
will  not  at  present  venture  to  enquire.* 

*  In  connection  with  the  investigation  of  the  radiation  and  absorption 
of  heat  by  gases  and  vapours,  it  gives  me  pleasure  to  refer  to  the  prompt 
and  intelligent  aid  rendered  me  by  Mr.  Becker,  of  the  firm  of  Elliotts',  30 
West  Strand. 

From  the  more  energetic  gases  and  vapours,  a  series  of  very  striking 
class  experiments  may  be  derived,  interesting  alike  to  the  chemist  and  the 
natural  philosopher.  Mr.  Becker  has  constructed  a  cheap  form  of  appa- 
ratus suitable  for  the  experiments.  Where  quantitative  results  are  not 
required,  two  cubes  of  hot  water,  an  open  tin  tube,  a  thermo-electric  pile, 
and  a  galvanometer,  magnetized,  as  described  in  the  Appendix  to  Lecture 
I.,  will  suffice  to  illustrate  the  action  of  the  stronger  gases  and  vapours. 
A  current  of  air  from  a  common  bellows  will  carry  the  vapour  into  the 
tube. 

The  fear  of  being  led  too  far  from  my  subject  causes  me  to  withhold 
all  speculation  as  to  the  cause  of  atmospheric  polarisation.  I  may,  how 
ever,  remark,  that  the  polarisation  of  heat  was  illustrated  by  means  of  the 
mica  piles  with  which  Professor  (now  Principal)  J.  D.  Forbes  first  suc- 
ceeded in  establishing  the  fact  of  polarisation. 


NOTE. 


(8)  Receiving  the  beam  from  the  electric  lamp  upon  the  polished  plate 
of  salt,  so  as  to  reflect  the  light  on  to  a  screen ;  and  placing  a  lens  in  front 
of  the  salt,  so  as  to  produce  an  image  of  its  polished  surface  on  the  screen ; 
on  breathing  against  the  salt  through  a  glass  tube,  beautiful  iridescences 
instantly  flash  forth,  which  may  be  seen  by  hundreds  at  once. 


APPENDIX  TO  LECTURE  XL 


EXTRACTS  FROM  A  DISCOURSE  '  ON  RADIATION  THROUGH  THE 
EARTH'S  ATMOSPHERE.' 

1  KOBODT  ever  obtained  the  idea  of  a  line  from  Euclid's  definition 
of  it—"  length  without  breadth."  The  idea  is  obtained  from  a 
real  physical  line,  drawn  by  a  pen  or  pencil,  and  therefore  pos- 
sessing width ;  this  idea  being  afterwards  brought,  by  a  process  of 
abstraction,  more  nearly  into  accordance  with  the  conditions  of 
the  definition.  So,  also,  with  regard  to  physical  phenomena;  we 
must  help  ourselves  to  a  conception  of  the  invisible,  by  means  of 
proper  images  derived  from  the  visible,  afterwards  purifying  our 
conceptions  to  the  needful  extent.  Definiteness  of  conception, 
even  though  at  some  expense  to  delicacy,  is  of  the  greatest  utility 
in  dealing  with  physical  phenomena.  Indeed,  it  may  be  questioned 
whether  a  mind  trained  in  physical  research  can  at  all  enjoy  peace, 
without  having  made  clear  to  itself  some  possible  way  of  conceiv- 
ing those  operations  which  lie  beyond  the  boundaries  of  sense,  and 
in  which  sensible  phenomena  originate. 

4  When  we  speak  of  radiation  through  the  atmosphere,  we  ought 
to  be  able  to  affix  definite  physical  ideas,  both  to  the  term  atmos- 
phere and  the  term  radiation.  It  is  well  known  that  our  atmos- 
phere is  mainly  composed  of  the  two  elements,  oxygen  and  nitro- 
gen. These  elementary  atoms  may  be  figured  as  small  spheres, 
scattered  thickly  in  the  space  which  immediately  surrounds  the 
earth.  They  constitute  about  99£  per  cent,  of  the  atmosphere. 
Mixed  with  these  atoms,  we  have  others  of  a  totally  different 
character ;  we  have  the  molecules,  or  atomic  groups,  of  carbonic 
acid,  of  ammonia,  and  of  aqueous  vapour.  In  these  substances 
diverse  atoms  have  coalesced,  forming  little  systems  of  atoms. 


416  APPENDIX   TO    LECTURE   XT. 

The  molecule  of  aqueous  vapour,  for  example,  consists  of  two  at- 
oms of  hydrogen,  united  to  one  of  oxygen ;  and  they  mingle,  as 
little  triads,  among  the  monads  of  oxygen  and  nitrogen  which  con- 
stitute the  great  mass  of  the  atmosphere. 

1  These  atoms  and  molecules  are  separate,  but  they  are  embraced 
by  a  common  medium.  "Within  our  atmosphere  exists  a  second, 
and  a  finer  atmosphere,  in  which  the  atoms  of  oxygen  and  nitro- 
gen hang  like  suspended  grains.  This  finer  atmosphere  unites  not 
only  atom  with  atom,  but  star  with  star ;  and  the  light  of  all  suns, 
and  of  all  stars,  is  in  reality  a  kind  of  music,  propagated  throng! i 
this  interstellar  air.  This  image  must  be  clearly  seized,  and  then 
we  have  to  advance  a  step.  We  must  not  only  figure  our  atoms 
suspended  in  this  medium,  but  vibrating  in  it.  In  this  motion  of 
the  atoms  consists  what  we  call  their  heat.  "  What  is  heat  in  us," 
as  Locke  has  perfectly  expressed  it,  "is  in  the  body  heated  noth- 
ing but  motion."  Well,  we  must  figure  this  motion  communicated 
to  the  medium  in  which  the  atoms  swing,  and  sent  in  ripples 
through  it,  with  inconceivable  velocity,  to  the  bounds  of  space. 
Motion  in  this  form,  unconnected  with  ordinary  matter,  but  speed- 
ing through  the  interstellar  medium,  receives  the  name  of  Radiant 
Heat;  and,  if  competent  to  excite  the  nerves  of  vision,  we  call  it 
Light. 

'Aqueous  vapour  was  defined  to  be  an  invisible  gas.  Vapour 
was  permitted  to  issue  horizontally  with  considerable  force  from 
a  tube  connected  with  a  small  boiler.  The  track  of  the  cloud  of 
condensed  steam  was  vividly  illuminated  by  the  electric  light. 
"What  was  seen,  however,  was  not  vapour,  but  vapour  condensed 
to  water.  Beyond  the  visible  end  of  the  jet,  the  cloud  resolved 
itself  into  true  vapour.  A  lamp  was  placed  under  the  jet,  at  va- 
rious points ;  the  cloud  was  cut  sharply  on0  at  that  point,  and  when 
the  flame  was  placed  near  the  efflux  orifice,  the  cloud  entirely  dis- 
appeared. The  heat  of  the  lamp  completely  prevented  precipita- 
tion. This  same  vapour  was  condensed  and  congealed  on  the  sur- 
face of  a  vessel  containing  a  freezing  mixture,  from  which  it  was 
scraped,  in  quantities  sufficient  to  form  a  small  snowball.  The 
beam  of  the  electric  lamp,  moreover,  was  sent  through  a  large  re- 
ceiver placed  on  an  air-pump.  A  single  stroke  of  the  pump  caused 
the  precipitation  of  the  aqueous  vapour  within,  which  became 
beautifully  illuminated  by  the  beam ;  while,  upon  a  screen  behind. 


DISCOURSE   ON  KADIATION.  417 

a  richly-coloured  halo,  due  to  diffraction  by  the  little  cloud  within 
the  receiver,  flashed  forth. 

'  The  Avaves  of  heat  speed  from  our  earth  through  the  atmos- 
phere towards  space.  These  waves  dash  in  their  passage  against 
the  atoms  of  oxygen  and  nitrogen,  and  against  the  molecules  of 
aqueous  vapour.  Thinly  scattered  as  these  latter  are,  we  might 
naturally  think  meanly  of  them,  as  barriers  to  the  waves  of  heat. 
We  might  imagine  that  the  wide  spaces  between  the  vapour  mole- 
cules would  be  an  open  door  for  the  passage  of  the  undulations ; 
and  that  if  those  waves  were  at  all  intercepted,  it  would  be  by  the 
substances  which  form  99£  per  cent,  of  the  whole  atmosphere. 
Three  or  four  years  ago,  however,  it  was  found  by  the  speaker  that 
this  small  modicum  of  aqueous  vapour  intercepted  fifteen  times 
the  quantity  of  heat  stopped  by  the  whole  of  the  air  in  which  it 
was  diffused.  It  was  afterwards  found  that  the  dry  air  then  ex- 
perimented with  was  not  perfectly  pure ;  and  that  the  purer  the 
air  became,  the  more  it  approached  the  character  of  a  vacuum,  and 
the  greater,  by  comparison,  became  the  action  of  the  aqueous  va- 
pour. The  vapour  was  found  to  act  with  30,  40,  50,  60,  70  times 
the  energy  of  the  air  in  which  it  was  diffused ;  and  no  doubt  was 
entertained  that  the  aqueous  vapour  of  the  air  which  filled  the 
Royal  Institution  theatre,  during  the  delivery  of  the  discourse,  ab- 
sorbed 90  or  100  times  the  quantity  of  radiant  heat  which  was  ab- 
sorbed by  the  main  body  of  the  air  of  the  room.  Looking  at  the 
single  atoms,  for  every  200  of  oxygen  and  nitrogen  there  is  about 
1  of  aqueous  vapour.  This  1  is  80  times  more  powerful  than  the 
200 ;  and  hence,  comparing  a  single  atom  of  oxygen  or  nitrogen 
with  a  single  atom  of  aqueous  vapour,  we  may  infer  that  the  action 
of  the  latter  is  16,000  times  that  of  the  former. 

'  No  doubt  can  exist  of  the  extraordinary  opacity  of  this  sub- 
stance to  the  rays  of  obscure  heat ;  particularly  such  rays  as  are 
emitted  by  the  earth,  after  being  wanned  by  the  sun.  Aqueous 
vapour  is  a  blanket,  more  necessary  to  the  vegetable  life  of  Eng- 
land than  clothing  is  to  man.  Eemove  for  a  single  summer-night 
the  aqueous  vapour  from  the  air  which  overspreads  this  country, 
and  you  would  assuredly  destroy  every  plant  capable  of  being 
destroyed  by  a  freezing  temperature.  The  warmth  of  our  fields 
and  gardens  would  pour  itself  unrequited  into  space,  and  the  sun 
would  rise  upon  an  island  held  fast  in  the  iron  grip  of  frost.  The 
18* 


418  APPENDIX  TO  LECTURE  xi. 

aqueous  vapour  constitutes  a  local  dam,  by  which  the  temperature 
at  the  earth's  surface  is  deepened :  the  dam,  however,  finally  over- 
flows, and  we  give  to  space  all  that  we  receive  from  the  sun. 

'  The  sun  raises  the  vapours  of  the  equatorial  ocean  ;  they  rise, 
but  for  a  time  a  vapour  screen  spreads  above  and  around  them. 
But  the  higher  they  rise,  the  more  they  come  into  the  presence 
of  pure  space ;  and  when,  by  their  levity,  they  have  penetrated 
the  vapour  screen,  which  lies  close  to  the  earth's  surface,  what 
must  occur  ? 

'  It  has  been  said  that,  compared  atom  for  atom,  the  absorption 
of  an  atom  of  aqueous  vapour  is  16,000  times  that  of  air.  Now 
the  power  to  absorb  and  the  power  to  radiate-  are  perfectly  recip- 
rocal and  proportional.  The  atom  of  aqueous  vapour  will  there- 
fore radiate  with  16,000  times  the  energy  of  an  atom  of  air.  Im- 
agine, then,  this  powerful  radiant  in  the  presence  of  space,  and 
with  no  screen  above  it  to  check  its  radiation.  Into  space  it 
pours  its  heat,  chills  itself,  condenses,  and  the  tropical  torrents 
are  the  consequence.  The  expansion  of  the  air,  no  doubt,  also 
refrigerates  it ;  but  in  accounting  for  deluges,  the  chilling  of  the 
vapour  by  its  own  radiation  must  play  a  most  important  part. 
The  rain  quits  the  ocean  as  vapour ;  returns  to  it  as  water.  How 
are  the  vast  stores  of  heat,  set  free  by  the  change  from  the  vapor- 
ous to  the  liquid  condition,  disposed  of?  Doubtless,  in  great  part, 
they  are  wasted  by  radiation  into  space.  Similar  remarks  apply 
to  the  cumuli  of  our  latitudes.  The  warmed  air,  charged  with 
vapour,  rises  in  columns,  so  as  to  penetrate  the  vapour  screen 
which  hugs  the  earth ;  in  the  presence  of  space,  the  head  of  each 
pillar  wastes  its  heat  by  radiation,  condenses  to  a  cumulus,  which 
constitutes  the  visible  capital  of  an  invisible  column  of  saturated 
air.  Numberless  other  meteorological  phenomena  receive  their 
solution  by  reference  to  the  radiant  and  absorbent  properties  of 
aqueous  vapour.' 

The  radiant  power  of  a  vapour  is  proportional  to  its  absorbent 
power.  Experiments  on  the  dynamic  radiation  of  dried  and  un- 
dried  air  prove  the  superiority  of  the  latter  as  a  radiator.  The 
following  experiment,  performed  by  Dr.  Frankland  in  the  theatre 
of  the  Royal  Institution,  showed  the  effect  to  a  large  audience. 
A  charcoal  chauffer,  14  inches  high  and  6  inches  in  diameter,  was 


HOOKEK   AND   LIVINGSTONE'S   OBSERVATIONS.  419 

placed  in  front  of  a  thermo-electric  pile,  and  at  a  distance  from  it 
of  two  feet.  The  radiation  from  the  chauffer  itself  was  inter- 
cepted by  a  metallic  screen.  The  deflection  due  to  the  radiation 
from  the  ascending  column  of  hot  carbonic  acid  was  then  carefully 
neutralised  by  a  constant  source  of  heat,  radiating  against  the  op- 
posite face  of  the  pile.  A  current  of  steam  was  then  forced  verti- 
cally through  the  chauffer.  The  deflection  of  the  galvanometer 
was  prompt  and  powerful.  When  the  current  of  steam  was  in- 
terrupted, the  needle  returned  to  zero.  "When,  instead  of  a  cur- 
rent of  steam,  a  current  of  air  was  forced  through  the  chauffer,  the 
slight  effect  produced  showed  the  pile  to  be  chilled  instead  of 
warmed.  In  this  experiment  Dr.  Frankland  compared  aqueous 
vapour,  not  with  air,  but  with  the  more  powerful  carbonic  acid, 
and  demonstrated  the  superiority  of  the  vapour  as  a  radiator.* 

The  following  remarkable  passage  from  Hooker's  *  Himalayan 
Journals,'  1st  edit.  vol.  ii.  p.  407,  also  bears  upon  the  present  sub- 
ject: 'From  a  multitude  of  desultory  observations  I  conclude 
that,  at  7,400  feet,  125-7°,  or  67°  above  the  temperature  of  the 
air,  is  the  average  effect  of  the  sun's  rays  on  a  black  bulb  ther- 
mometer. .  .  .  These  results,  though  greatly  above  those 
obtained  at  Calcutta,  are  not  much,  if  at  all,  above  what  may  be 
observed  on  the  plains  of  India.  The  effect  is  much  increased  by 
elevation.  At  10,000  feet,  in  December,  at  9  a.  m.,  I  saw  the  mer- 
cury mount  to  132°,  while  the  temperature  of  shaded  snow  hard 
by  was  22°.  At  13,100  feet,  in  January,  at  9  a.  m.,  it  has  stood 
at  98°,  with  a  difference  of  68'2°,  and  at  10  a.  m.  at  114°,  with  a 
difference  of  81*4°,  whilst  the  radiating  thermometer  on  the  snow 
had  fallen  at  sunrise  to  0'7°.' 

These  enormous  differences  between  the  shaded  and  the  un- 
shaded air,  and  between  the  air  and  the  snow,  are,  no  doubt,  due 
to  the  comparative  absence  of  aqueous  vapour  at  these  elevations. 
The  air  is  incompetent  to  check  either  the  solar  or  the  terrestrial 
radiation,  and  hence  the  maximum  heat  in  the  sun  and  the  maxi- 
mum cold  in  the  shade  must  stand  very  wide  apart.  The  differ- 
ence between  Calcutta  and  the  plains  of  India  is  accounted  for  in 
the  same  way. 

Dr.  Livingstone,  in  his  'Travels  in  South  Africa,'  has  given 
some  striking  examples  of  the  difference  in  nocturnal  chilling 

*  Phil  Mag.  vol.  xxvii.  p.  326. 


4:20  APPENDIX   TO   LECTURE   XT. 

when  the  air  is  dry  and  when  laden  with  moisture.  Tims  he 
finds  in  South  Central  Africa  during  the  month  of  June,  'the 
thermometer  early  in  the  mornings  at  from  42°  to  52° ;  at  noon, 
94°  to  96°,  or  a  mean  difference  of  48°  between  sunrise  and  mid- 
day. The  range  would  probably  have  been  found  still  grcatei 
had  not  the  thermometer  been  placed  in  the  shade  of  his  tent, 
which  was  pitched  under  the  thickest  tree  he  could  find.  He 
adds,  moreover,  '  the  sensation  of  cold  after  the  heat  of  the  day 
was  very  keen.  The  Balonda  at  this  season  never  leave  their  fires 
till  nine  or  ten  in  the  morning.  As  the  cold  was  so  great  here,  it 
was  probably  frosty  at  Linyanti ;  I  therefore  feared  to  expose  my 
young  trees  there.'* 

Dr.  Livingstone  afterwards  crosses  the  continent  and  reaches 
the  river  Zambesi  at  the  beginning  of  the  year.  Here  the  ther- 
mometric  range  is  reduced  from  48°  to  12°.  He  thus  describes 
the  change  he  felt  on  entering  the  valley  of  the  river :  '  We  were 
struck  by  the  fact,  that  as  soon  as  we  came  between  the  range  of 
hills  which  flank  the  Zambesi,  the  rains  felt  warm.  At  sunrise 
the  thermometer  stood  at  from  82°  to  86°  ;  at  midday,  in  the  cool- 
est shade,  namely,  in  my  little  tent,  under  a  shady  tree,  at  96°  to 
98° ;  and  at  sunset  at  86°.  This  is  different  from  anything  we 
experienced  in  the  interior.'  t 

Proceeding  towards  the  mouth  of  the  river,  on  January  16  he 
makes  the  following  additional  observation :  '  The  Zambesi  is  very 
broad  here  (at  Zumbo),  but  contains  many  inhabited  islands. 
"VVe  slept  opposite  one  on  the  16th,  called  Shibanga.  The  nights 
are  warm,  the  temperature  never  falling  below  80° ;  it  was  91° 
even  at  sunset.  One  cannot  cool  the  water  by  a  wet  towel  round 
the  vessel.  .  .  .'{ 

In  Central  Australia  the  daily  range  of  the  thermometer  is 
still  greater.  The  following  extract  is  from  a  paper  by  Mr.  "W.  S. 
Jevons  '  On  some  Data  concerning  the  Climate  of  Australia  and 
New  Zealand ' :  ' .  .  .  In  the  interior  of  the  continent  of  Aus- 
tralia the  fluctuations  of  temperature  are  immensely  increased. 
The  heat  of  the  air,  as  described  by  Captain  Sturt,  is  fearful 
during  summer;  thus,  in  about  lat.  30°  50'  S.,  and  Ion.  141°  18' 
E.,  he  writes:  "The  thermometer  every  day  rose  to  112°  or 
116°  in  the  shade,  whilst  in  the  direct  rays  of  the  sun  from  140° 

*  Livingstone's  Travels,  p.  484.     f  Ibid.  p.  575.     £  Ibid.  p.  589. 


THEEMOHETBIO   BANGE   IN   AUSTEALIA.  421 

to  150V  Again,  "  at  a  quarter  past  three  p.  m.  on  January  21 
(1845),  the  thermometer  had  risen  to  181°  in  the  shade,  and  to 
154°  in  the  direct  rays  of  the  sun."  ...  In  the  winter  the 
thermometer  was  observed  as  low  as  24°,  giving  an  extreme  range 
of  107°. 

'  The  fluctuations  of  temperature  were  often  very  great  and 
sudden,  and  were  severely  felt.  On  one  occasion  (October  25), 
the  temperature  rose  to  110°  during  the  day,  but  a  squall  coming 
on,  it  fell  to  38°  at  the  following  sunrise ;  it  thus  varied  72°  in  less 
than  twenty-four  hours.  .  .  .  Mitchell,  on  his  last  journey  to 
the  1ST.  W.  interior,  had  very  cold  frosty  nights.  On  May  22,  the 
thermometer  stood  at  12°  in  the  open  air.  .  .  .  Still,  in  the 
day  time,  the  air  was  warm,  and  the  daily  range  of  temperature 
was  enormous.  Thus,  on  June  2,  the  thermometer  rose  from  11° 
at  sunrise  to  67°  at  four  p.m.;  or  through  a  range  of  56°.  On 
June  12,  the  range  was  53°,  and  on  many  other  days  nearly  as 
great. 

Even  at  Sydney  the  average  daily  range  of  the  thermometer  is 
21°,  whilst  at  Greenwich  the  average  range  is  only  17°.  '  It  thus 
appears  that  even  close  to  the  ocean  the  mean  daily  range  of  the 
Australian  climate  is  very  considerable.  It  is  least  in  the  autumn 
and  greatest  during  the  cloudless  days  of  spring.'  After  giving  a 
table  of  the  seasonal  variation  of  the  rainfall  in  Australia,  Mr.  Jev- 
ons  remarks  that  '  it  is  plainly  shown  that  the  most  rainy  season 
of  the  year  on  the  east  coast  is  the  autumn,  that  is,  the  three 
months,  March,  April,  May.  The  spring  season  appears  the  driest, 
summer  and  winter  being  intermediate.' 

"Without  quitting  Europe,  we  find  places  where,  while  the  day 
temperature  is  very  high,  the  hour  before  sunrise  is  intensely  cold. 
I  have  often  experienced  this  in  the  post-wagens  of  Germany ;  and 
I  am  informed  that  the  Hungarian  peasants,  if  exposed  at  night, 
take  care,  even  in  hot  weather,  to  protect  themselves  by  heavy 
cloaks  against  the  nocturnal  chill.  The  observations  of  MM.  Bra- 
vais  and  Martins  on  the  Grand  Plateau  of  Mont  Blanc  have  been 
already  referred  to.  M.  Martins  has  recently  added  to  our  knowl- 
edge by  making  observations  on  the  heating  of  the  soil  at  great 
elevations,  and  finds  on  the  summit  of  the  Pic  du  Midi  the  heat  of 
the  soil  exposed  to  the  sun,  above  that  of  the  air,  to  bo  twice  as 
great  as  in  the  valley  at"  the  base  of  the  mountain.  '  The  immense 


4:22  APPENDIX   TO   LECTUKE   XI. 

heating  of  the  soil,'  writes  M.  Martins,  '  compared  with  that  of  the 
air  on  high  mountains,  is  the  more  remarkable,  since,  during  the 
nights,  the  cooling  by  radiation  is  there  much  greater  than  in  the 
plain.'  The  observations  of  the  Messrs.  Schlagentweit  furnish,  if  I 
mistake  not,  many  illustrations  of  the  action  of  aqueous  vapour ; 
and  I  do  not  doubt,  that  the  more  this  question  is  tested,  the  more 
clearly  will  it  appear  that  the  radiant  and  absorbent  powers  of  this 
substance  enable  it  to  play  a  most  important  part  in  the  phenom- 
ena of  meteorology. 


LECTURE    XII. 

ABSORPTION  OP  HEAT  BY  VOLATILE  LIQUIDS — ABSORPTION  OF  HEAT  BY  THE  VA- 
POURS OF  THOSE  LIQUIDS  AT  A  COMMON  PRESSURE — ABSORPTION  OF  HEAT 
BY  THE  SAME  VAPOURS  WHEN  THE  QUANTITIES  OF  VAPOUR  ARE  PROPOR- 
TIONAL TO  THE  QUANTITIES  OF  LIQUID — COMPARATIVE  VIEW  OF  THE  AC- 
TION OF  LIQUIDS  AND  THEIR  VAPOURS  UPON  RADIANT  HEAT — PHYSICAL 

CAUSE   OF   OPACITY  AND    TRANSPARENCY INFLUENCE    OF   TEMPERATURE  ON 

THE  TRANSMISSION  OF  RADIANT  HEAT — CHANGES  OF  POSITION  THROUGH 
CHANGES  OF  TEMPERATURE — RADIATION  FROM  FLAMES — INFLUENCE  OF  OS- 
CILLATING PERIOD  ON  THE  TRANSMISSION  OF  PvADIANT  HEAT — EXPLANATION 
OF  RESULTS  OF  MELLONI  AND  KNOBLAUCH. 

THE  natural  philosophy  of  the  future  must,  I  imagine, 
mainly  consist  in  the  investigation  of  the  relations 
subsisting  between  the  ordinary  matter  of  the  universe 
and  the  ether  in  which  this  matter  is  immersed.  Regard- 
ing the  motions  of  the  ether  itself,  the  optical  investiga- 
tions of  the  last  half  century  have  left  nothing  to  be  de- 
sired; but  regarding  the  atoms  and  molecules,  whence 
issue  the  undulations  of  light  and  heat,  and  their  relations 
to  the  medium  in  which  they  move,  and  by  which  they 
are  set  in  motion,  these  investigations  teach  us  little.  To 
come  closer  to  the  origin  of  the  ethereal  waves — to  obtain, 
if  possible,  some  experimental  hold  of  the  oscillating  at- 
oms themselves — has  been  the  main  object  of  those  re- 
searches on  the  radiation  and  absorption  of  heat  by  gases 
and  vapours,  which,  in  brief  outline,  I  have  sketched  be- 
fore you. 

These  enquiries  have  made  known  the  differences 
which  exist  between  different  gaseous  molecules,  as  re- 
gards their  power  of  emitting  and  absorbing  radiant  heat. 


424  LECTUKE   XII. 

When  a  gas  is  condensed  to  a  liquid,  the  molecules  ap- 
proach and  grapple  with  each  other,  by  forces  which  are 
insensible  as  long  as  the  gaseous  state  is  maintained.  But 
though  thus  condensed  and  enthralled,  the  all-pervading 
ether  still  surrounds  the  molecules.  If,  then,  the  power 
of  radiation  and  absorption  depend  upon  them  individ- 
ually, we  may  expect  that  the  deportment  towards  radiant 
heat  of  the  free  molecule,  will  maintain  itself  after  that 
molecule  has  relinquished  its  freedom  and  formed  part  of 
a  liquid.  If,  on  the  other  hand,  the  state  of  aggregation 
be  of  paramount  importance,  we  may  expect  to  find,  on 
the  part  of  liquids,  a  deportment  altogether  different  from 
that  of  their  vapours.  Which  of  these  views  corresponds 
with  the  truth  of  nature,  we  have  now  to  enquire. 

Melloni  examined  the  diathermancy  of  various  liquids, 
but  he  employed  for  this  purpose  the  flame  of  an  oil-lamp, 
covered  by  a  glass  chimney.  His  liquids,  moreover,  were 
contained  in  glass  cells;  hence,  the  radiation  was  pro- 
foundly modified  before  it  entered  the  liquid  at  all,  glass 
being  impervious  to  a  considerable  part  of  the  emission. 
In  the  examination  of  the  question  now  before  us,  it  was 
my  wish  to  interfere  as  little  as  possible  with  the  primi- 
tive emission,  and  an  apparatus  was  therefore  devised  in 
which  a  layer  of  liquid,  of  any  thickness,  could  be  enclosed 
between  two  polished  plates  of  rocksalt. 

The  apparatus  consists  of  the  following  parts : — A  B  c 
(fig.  A)  is  a  plate  of  brass,  3*4  inches  long,  2'1  inches  wide, 
and  0*3  of  an  inch  thick.  Into  it,  at  its  corners,  are  rig- 
idly fixed  four  upright  pillars,  furnished  at  the  top  with 
screws,  for  the  reception  of  the  nuts  q  r  s  t.  T>  E  F  is  a 
second  plate  of  brass,  of  the  same  size  as  the  former,  and 
pierced  with  holes  at  its  four  corners,  so  as  to  enable  it  to 
slip  over  the  four  columns  of  the  plate  ABC.  Both  these 
plates  are  perforated  by  circular  apertures,  m  n  and  op, 
1*35  inch  in  diameter.  G  n  i  is  a  third  plate  of  brass,  of 


ROCKSALT   CELLS. 


425 


the  same  area  as  D  E  F,  and,  like  it,  having  its  centre 
and  its  corners  perforated.     The  plate  G  n  i  is  intended  to 

Fm.  A. 


separate  -the  two  plates  of  rocksalt  which  are  to  form  the 
walls  of  the  cell,  and  its  thickness  determines  that  of  the 
liquid  layer.  The  separating  plate  G  H  i  was  ground  with 
the  utmost  accuracy,  and  the  surfaces  of  the  plates  of  salt 
were  polished  with  extreme  care,  with  a  view  to  render- 
ing the  contact  between  the  salt  and  the  brass  water-tight. 
In  practice,  however,  it  was  found  necessary  to  introduce 
washers  of  thin  letter-paper  between  the  plates  of  salt  and 
the  separating  plate. 

In  arranging  the  cell  for  experiment,  the  nuts  q  r  s  t 


4:26 


LECTUKE    XH. 


are  unscrewed,  and  a  washer  of  india-rubber  is  first  placed 
on  A  B  c.  On  this  washer  is  placed  one  of  the  plates  of 
rocksalt.  On  the  plate  of  rocksalt  is  laid  the  washer  of 
letter-paper,  and  on  this  again  the  separating  plate  G  H  I. 

FIG.  B. 


A  second  washer  of  paper  is  placed  on  this  plate,  then 
comes  the  second  plate  of  salt,  on  which  another  india- 
rubber  washer  is  laid.  The  plate  D  E  F  is  finally  slipped 
over  the  columns,  and  the  whole  arrangement  is  tightly 
screwed  together  by  the  nuts  q  r  s  t.  Thus,  when  the 
plates  of  rocksalt  are  in  position,  a  cylinder,  as  long  as  the 
plate  G  H  i  is  thick,  is  enclosed  between  them,  and  this 
space  can  be  filled  with  any  liquid  through  the  orifice  fc. 


PLATINUM   LAMP.  427 

The  use  of  the  india-rubber  washers  is  to  relieve  the  crush, 
ing  pressure  which  would  be  applied  to  the  plates  of  salt, 
if  they  were  in  actual  contact  with  the  brass ;  and  the  use 
of  the  paper  washers  is,  as  already  explained,  to  render 
the  cell  liquid-tight.  After  each  experiment,  the  appa- 
ratus is  unscrewed,  the  plates  of  salt  are  removed  and 
thoroughly  cleansed ;  the  cell  is  then  remounted,  and  in 
two  or  three  minutes  all  is  ready  for  a  new  experiment. 

My  next  necessity  was  a  perfectly  steady  source  of 
heat,  of  sufficient  intensity  to  penetrate  the  most  absorb- 
ent of  the  liquids  to  be  subjected  to  examination.  This 
was  found  in  a  spiral  of  platinum  wire,  rendered  incandes- 
cent by  an  electric  current.  The  frequent  use  of  this 
source  led  to  the  construction  of  the  lamp  shown  in  fig.  B. 
A  is  a  globe  of  glass  three  inches  in  diameter,  fixed  upon 
a  stand,  which  can  be  raised  and  lowered.  At  the  top  of 
the  globe  is  an  opening,  into  which  a  cork  is  fitted,  and 
through  the  cork  pass  two  wires,  the  ends  of  which  are 
united  by  the  platinum  spiral  s.  The  wires  are  carried 
down  to  the  binding  screws  a  5,  which  are  fixed  in  the 
foot  of  the  stand,  so  that  when  the  instrument  is  attached 
to  the  battery,  no  strain  is  ever  exerted  on  the  wires  which 
carry  the  spiral.  The  ends  of  the  thick  wire  to  which  the 
spiral  is  attached  are  also  of  stout  platinum,  for  when  it 
was  attached  to  copper  wires  unsteadiness  was  introduced 
through  oxidation.  The  heat  issues  from  the  incandescent 
spiral  by  the  opening  d,  which  is  an  inch  and  a  half  in 
diameter.  Behind  the  spiral,  finally,  is  a  metallic  reflector, 
r,  which  augments  the  flux  of  heat  without  sensibly  chang- 
ing its  quality.  In  the  open  air  the  red-hot  spiral  is  a 
capricious  source  of  heat,  but  surrounded  by  its  glass 
globe  its  steadiness  is  admirable. 

The  whole  experimental  arrangement  will  be  imme- 
diately understood  from  the  sketch  given  in  fig.  C.  A  is 
the  platinum  lamp  just  described,  heated  by  a  current 


428 


LECTUKE   XII. 


EXPERIMENTAL   AEKANGEMENT.  429 

from  a  Grove's  battery  of  five  cells.  It  is  necessary  that 
this  lamp  should  remain  perfectly  constant  throughout  the 
day;  arid  to  keep  it  so,  a  tangent  galvanometer  and  a 
rheocord  are  introduced  into  the  circuit. 

In  front  of  the  spiral,  and  with  an  interior  reflecting 
surface,  is  the  tube  B,  through  which  the  heat  passes  to 
the  rocksalt  cell  c.  This  cell  is  placed  on  a  little  stage, 
soldered  to  the  back  of  the  perforated  screen  s  s',  so  that 
the  heat,  after  having  crossed  the  cell,  passes  through  the 
hole  in  the  screen,  and  afterwards  impinges  on  the  thermo- 
electric pile  p.  The  pile  is  placed  at  some  distance  from 
the  screen  s  s',  so  as  to  render  the  temperature  of  the  cell 
c  itself  of  no  account,  c'  is  the  compensating  cube,  con- 
taining water  kept  boiling  by  steam  from  the  pipe  p.  Be- 
tween the  cube  c'  and  the  pile  p  is  the  screen  Q,  which 
regulates  the  amount  of  heat  falling  on  the  posterior  face 
of  the  pile.  The  whole  arrangement  is  here  exposed,  but, 
in  practice,  the  pile  P  and  the  cube  c'  are  carefully  pro- 
tected from  the  capricious  action  of  the  surrounding  air. 

The  experiments  are  thus  performed.  The  empty 
rocksalt  cell  c  being  placed  on  its  stage,  a  double  silvered 
screen  (not  shown  in  the  figure)  is  first  introduced  be- 
tween the  end  of  the  tubes  and  the  cell  c;  the  spiral 
being  thus  totally  cut  off,  and  the  pile  subjected  to  the 
action  of  the  cube  c'  alone.  By  means  of  the  screen  Q, 
the  heat  received  by  the  pile  from  c,  is  reduced  until  the 
total  heat  to  be  adopted  throughout  the  series  of  experi- 
ments is  obtained :  say,  that  it  is  sufficient  to  produce  a 
galvanometric  deflection  of  50  degrees.  The  double  screen 
used  to  intercept  the  radiation  from  the  spiral  is  then 
gradually  withdrawn,  until  this  radiation  completely  neu- 
tralises that  from  the  cube  c',  and  the  needle  of  the 
galvanometer  points  steadily  to  zero.  The  position  of 
the  double  screens,  once  ilxed,  remains  subsequently  un- 
changed, the  slight  and  slow  alteration  of  the  source  being 


430 


LECTURE   XH. 


neutralised  by  the  rlieocord.  Thus,  the  rays  in  the  first 
instance  pass  from  the  spiral  through  the  empty  rocksalt 
cell.  A  small  funnel,  supported  by  a  suitable  stand,  dips 
into  the  aperture  which  leads  into  the  cell,  and  through 
this  the  liquid  is  poured.  The  introduction  of  the  liquid 
destroys  the  previous  equilibrium,  the  galvanometer  needle 
moves,  and  finally  assumes  a  steady  deflection.  From 
this  deflection  we  can  immediately  calculate  the  quantity 
of  heat  absorbed  by  the  liquid,  and  express  it  in  hun- 
dredth s  of  the  entire  radiation. 

The  experiments  were  executed  with  eleven  different 
liquids,  employing  each  liquid  in  five  different  thicknesses. 
The  results  are  collected  together  in  the  following  table : — 

ABSORPTION  OF  HEAT  BY  LIQUIDS.     SOURCE  OF  HEAT:  PLATINUM  SPIRAL 
RAISED  TO  BRIGHT  REDNESS  BY  A  VOLTAIC  CURRENT. 


Liquid 

Thickness  of  liquid  in  parts  of  an  inch. 

0-02 

0-04 

0-07 

0-14 

0-27 

Bisulphide  of  carbon 
Chloroform    . 
Iodide  of  methyl 
Iodide  of  ethyl      . 

5-5 
16-6 
361 
38-2 
43-4 
58-3 
63-3 

65-2 
67-3 

80-7 

8-4 
25-0 
46-5 
50-7- 
65-7 
65-2 
73-5 
74-0 
76-3 
78-6 
86-1 

12-5 
35-0 
63-2 
59-0 
62-5 
73-6 
76-1 
78-0 
79-0 
83-6 
88-8 

15-2 
40-0 
65-2 
69-0 
71-5 
77-7 
78-6 
82-0 
84-0 
85-3 
•91-0 

17-3 
44-8 
68-6 
71-5 
73-6 
82-3 
85-2 
86-1 
87-0 
89-1 
91-0 

Amlyene 
Sulphuric  ether 
Acetic  ether  . 
Formic  ether 
Alcohol 
Water  . 

Here,  for  a  thickness  of  0'02  of  an  inch  we  find  the 
absorption  varying  from  a  minimum  of  5 '5  per  cent,  in  the 
case  of  bisulphide  of  carbon,  to  a  maximum  of  80*7  per 
cent,  in  the  case  of  water.  The  bisulphide  therefore  trans- 
mits 94-5  per  cent.,  while  the  water — a  liquid  equally 
transparent  to  light — transmits  only  19'3  per  cent,  of  the 
entire  radiation.  At  all  thicknesses,  water,  it  will  be  ob- 


ABSORPTION  BY   VOLATILE   LIQUIDS.  431 

served,  asserts  its  predominance.  Next  to  it,  as  an  ab- 
sorbent, stands  alcohol ;  a  body  which  also  resembles  it 
chemically. 

As  liquids,  then,  those  bodies  are  shown  to  possess 
very  different  capacities  of  intercepting  the  heat  emitted 
by  our  radiating  source ;  and  we  have  next  to  enquire 
whether  these  differences  continue,  after  the  molecules 
have  been  released  from  the  bond  of  cohesion.  We  must, 
of  course,  test  the  vapours  by  waves  of  the  same  period 
as  those  applied  to  the  liquids,  and  this  our  mode  of  ex- 
periment renders  easy  of  accomplishment.  The  heat  gen- 
erated in  a  wire  by  a  current  of  a  given  strength  being 
invariable,  it  was  only  necessary,  by  means  of  the  tangent 
compass  and  rheocord,  to  keep  the  current  constant  from 
day  to  day,  in  order  to  obtain,  both  as  regards  quantity 
and  quality,  an  invariable  source  of  heat. 

The  liquids  from  which  the  vapours  were  derived  were 
placed  in  small  long  flasks,  a  separate  flask  being  devoted 
to  each.  The  air  above  the  liquid,  and  within  it,  being 
first  carefully  removed  by  an  air-pump,  the  flask  was  at- 
tached to  the  experimental  tube,  in  which  the  vapours 
were  to  be  examined.  This  tube  was  of  brass,  49'6  inches 
long,  and  2 -4  inches  in  diameter,  its  two  ends  being 
stopped  by  plates  of  rocksalt.  Its  interior  surface  was 
polished.  With  the  single  exception  that  the  source  of 
heat  was  a  red-hot  platinum  spiral,  instead  of  a  plate  of 
copper,  the  arrangement  was  that  figured  in  Plate  I.  At 
the  commencement  of  each  experiment,  the  brass  tube 
being  thoroughly  exhausted,  and  the  radiation  from  the 
spiral  being  neutralised  by  that  from  the  compensating 
cube,  the  needle  stood  at  zero.  The  cock  of  the  flask  con- 
taining the  volatile  liquid  was  then  carefully  turned  on, 
and  the  vapour  allowed  slowly  to  enter  the  experimental 
tube.  When  a  pressure  of  0-5  of  an  inch  was  obtained, 
the  vapour  was  cut  off,  and  the  permanent  deflection  of 


432  LECTUEE   XII. 

the  needle  noted.  Knowing  the  total  heat,  the  absorp- 
tion in  lOOths  of  the  entire  radiation  could  be  at  once  de- 
duced from  the  deflection.  The  following  table  contains 
the  results : — 

RADIATION  OP  HEAT  THROUGH  VAPOURS.    SOURCE  :  RED-HOT  PLATINUM 
SPIRAL.    PRESSURE,  0*5  OF  AN  INCH. 

Absorption  per  cent. 

Bisulphide  of  carbon  .  .        4€7 

Chloroform   .  .  .  .6*5 

Iodide  of  methyl        .  .  .9-6 

Iodide  of  ethyl          .  .  .       17'7 

Benzol  ....       20'6 

Amylene  ....  27'5 
Alcohol  ....  28-1 
Formic  ether  .  .  .31-4 

Sulphuric  ether         .  .  .31-9 

Acetic  ether  ....  34*6 
Total  heat  ....  100-0 

We  are  now  in  a  condition  to  compare  the  action  of  a 
series  of  volatile  liquids,  with  that  of  the  vapours  of  those 
liquids,  upon  radiant  heat. 

Commencing  with  the  substance  of  the  lowest  absorp- 
tive energy,  and  proceeding  to  the  highest,  we  have  the 
following  orders  of  absorption : — 

Liquids  Vapours 

Bisulphide  of  carbon.  Bisulphide  of  carbon. 

Chloroform.  Chloroform. 

Iodide  of  methyl.  Iodide  of  methyl. 

Iodide  of  ethyl.  Iodide  of  ethyl. 

Benzol.  Benzol. 

Amylene.  Amylene. 

Sulphuric  ether.  Alcohol. 

Acetic  ether.  Formic  ether. 

Formic  ether.  Sulphuric  ether. 

Alcohol.  Acetic  ether. 
Water. 


OEDEK  OF  ABSORPTION  OF  LIQUIDS  AND  VAPOURS.      433 

Here,  as  far  as  amylene,  the  order  of  absorption  is  the 
same  for  both  liquids  and  vapours.  But  from  amylene 
downwards,  though  strong  liquid  absorption  is,  in  a  gen- 
eral way,  paralleled  by  strong  vapour  absorption,  the 
order  of  both  is  not  the  same.  There  is  not  the  slightest 
doubt  that,  next  to  water,  alcohol  is  the  most  powerful 
absorber  in  the  list  of  liquids ;  but  there  is  just  as  little 
doubt  that  the  position  which  it  occupies  in  the  list  of 
vapours  is  the  correct  one.  This  has  been  established  by 
reiterated  experiments.  Acetic  ether,  on  the  other  hand, 
though  certainly  the  most  energetic  absorber  in  the  state 
of  vapour,  falls  behind  both  formic  ether  and  alcohol  in 
the  liquid  state.  Still,  on  the  whole,  I  think  it  is  impossi- 
ble to  contemplate  these  results,  without  arriving  at  the 
conclusion  that  the  act  of  absorption  is,  in  the  main,  molec- 
ular, and  that  the  molecules  maintain  their  power  as 
absorbers  and  radiators  when  they  change  their  state  of 
aggregation.  Should  any  doubt,  however,  linger  as  to 
the  correctness  of  this  conclusion,  it  will  speedily  disap- 
pear. 

A  moment's  reflection  will  show  that  the  comparison 
here  instituted  is  not  a  strict  one.  We  have  taken  the 
liquids  at  a  common  thickness,  and  the  vapours  at  a  com- 
mon volume  and  pressure.  But  if  the  layers  of  liquid 
employed  were  turned,  bodily,  into  vapour,  the  volumes 
obtained  would  not  be  the  same.  Hence,  the  quantities 
of  matter  traversed  by  the  radiant  heat  are  neither  equal 
nor  proportional  to  each  other  in  the  two  cases,  and  to 
render  the  comparison  strict,  they  ought  to  be  propor- 
tional. It  is  easy,  of  course,  to  make  them  so ;  for  the 
liquids  being  examined  at  a  constant  volume,  their  spe- 
cific gravities  give  us  the  relative  quantities  of  matter 
traversed  by  the  radiant  heat,  and  from  these,  and  the 
vapour-densities,  we  can  immediately  deduce  the  corre- 
sponding volumes  of  the  vapour.  Dividing,  in  fact,  the 
19 


434  LEOTUEE  XH. 

specific  gravities  of  our  liquids  by  the  densities  of  their 
vapours,  we  obtain  the  following  series  of  vapour  vol- 
umes, whose  weights  are  proportional  to  the  masses  of 
liquid  employed : — 

TABLE  OP  PROPORTIONAL  VOLUMES.      l 

Bisulphide  of  carbon     .  .  .  0'48 

Chloroform        .  .  .  0*36 

Iodide  of  methyl  .  .  .  0'46 

Iodide  of  ethyl  .  .  .  0'36 

Benzol  .....  0'32 

Amylene  ....  0*26 

Alcohol  .  .  .  •         .  0-50 

Sulphuric  ether  .  .  .  0'28 

Formic  ether     .  .  .  .  0-36 

Acetic  ether      ....  0-29 

Water   .....  1-60 

Introducing  the  vapours,  in  the  volumes  here  indicated, 
into  the  experimental  tube,  the  following  results  were  ob- 
tained : — 

RADIATION  OP  HEAT  THROUGH  VAPOURS.    QUANTITY  OF  VAPOUR  PROPOR- 
TIONAL TO  THAT  OP  LIQUID. 

Name  of  Vapour  Pressure  in  parts    Absorption 

of  an  inch  percent. 

Bisulphide  of  carbon            .            .  0-48  4-3 

Chloroform  ....  0-36  6'6 

Iodide  of  methyl       .            .            .  0-46  10-2 

Iodide  of  ethyl          .            .            .  0'36  15'4 

Benzol          ....  0'32  16-8 

Amylene       ....  0-26  19-0 

Sulphuric  ether         .            .            .  0'28  21*5 

Acetic  ether             .           .            .  0-29  22-2 

Formic  ether            .            .            .  0-36  22*5 

Alcohol        ....  0-50  22-7 

Arranging  both  liquids  and  vapours  in  the  order  of 
their  absorption,  we  now  obtain  the  following  result : — 


CAUSE   OF   TRANSPAKENCY  AND   OPACITY.  i35 

Liquids  Vapours 

Bisulphide  of  carbon.  Bisulphide  of  carbon 

Chloroform.  Chloroform. 

Iodide  of  methyl.  Iodide  of  methyl. 

Iodide  of  ethyl.  Iodide  of  ethyl. 

Benzol.  Benzol. 

Amylene.  Amylene. 

Sulphuric  ether.  Sulphuric  ether. 

Acetic  ether.  Acetic  ether. 

Formic  ether.  Formic  ether. 

Alcohol.  Alcohol. 
Water.  * 

Here  the  discrepancies  revealed  by  our  former  series 
of  experiments  entirely  disappear,  and  it  is  proved  that 
for  heat  of  the  same  quality,  the  order  of  absorption  for 
liquids  and  their  vapours  is  the  same.  We  may,  there- 
fore, safely  infer  that  the  position  of  a  vapour,  as  an  ab- 
sorber or  radiator,  is  determined  by  that  of  the  liquid 
from  which  it  is  derived.  Granting  the  validity  of  this 
inference,  the  position  of  water  fixes  that  of  aqueous  va- 
pour. But  we  have  found  that,  for  all  thicknesses,  water 
exceeds  the  other  liquids  in  the  energy  of  its  absorption. 
Hence,  if  no  single  experiment  on  the  vapour  of  water 
existed,  we  should  be  compelled  to  conclude,  from  the  de- 
portment of  its  liquid,  that,  weight  for  weight,  aqueous 
vapour  transcends  all  others  in  absorptive  power.  Add 
to  this  the  direct  and  multiplied  experiments,  by  which 
the  action  of  this  substance  on  radiant  heat  has  been  es- 
tablished, and  we  have  before  us  a  body  of  evidence  suffi- 
cient, I  trust,  to  set  this  question  for  ever  at  rest,  and  to 
induce  the  meteorologist  to  apply  the  result,  without  mis- 
giving, to  the  phenomena  of  his  science. 

We  must  now  prepare  the  way  for  the  consideration 
of  an  important  question.  A  pendulum  swings  at  a  cer- 

*  Aqueous  vapour,  unmixed  with  air,  condenses  so  readily  that  it  can- 
not be  directly  examined  in  our  experimental  tube. 


436  LECTURE   XH. 

tain  definite  rate,  which  depends  upon  the  length  of  the 
pendulum.  A  spring  will  oscillate  at  a  rate  which  de- 
pends upon  the  weight  and  elastic  force  of  the  spring.  If 
we  coil  wire  into  a  long  spiral,  and  attach  a  bullet  to  the 
end,  the  bullet  will  oscillate  up  and  down,  at  a  rate  which 
depends  upon  its  weight,  and  upon  the  elasticity  of  the 
spiral.  A  musical  string,  in  like  manner,  has  its  determi- 
nate rate  of  vibration,  which  depends  upon  its  length, 
weight,  and  tension.  A  beam  which  bridges  a  gorge  has 
also  its  own  rate  of  oscillation ;  and  we  can  often,  by 
timing  our  movements  on  such  a  beam,  so  accumulate  the 
impulses  as  to  endanger  its  safety.  Soldiers,  in  crossing 
pontoon  bridges,  tread  irregularly,  lest  the  motion  im- 
parted to  the  pontoons  should  accumulate  to  a  dangerous 
extent.  The  step  of  persons  who  carry  water  on  their 
heads  in  open  pails  sometimes  coincides  with  the  oscilla- 
tion of  the  water  from  side  to  side  of  the  vessel,  until,  im- 
pulse being  added  to  impulse,  the  liquid  finally  splashes 
over  the  rim.  The  water  carrier  instinctively  alters  step, 
and  thus  reduces  the  liquid  to  comparative  tranquillity. 
These  ordinary  mechanical  facts  will  help  us  to  an  insight 
of  the  more  subtle  phenomena  of  light  and  radiant  heat. 
You  have  heard  a  particular  pane  of  glass  respond  to  a 
particular  note  of  an  organ ;  if  you  open  a  piano,  and  sing 
into  it,  some  one  string  will  also  respond.  Now,  in  the 
case  of  the  organ  the  pane  responds,  because  its  period  of 
vibration  happens  to  coincide  with  the  period  of  the  so- 
norous waves  that  impinge  upon  it ;  and  in  the  case  of  the 
piano,  that  string  responds  whose  period  of  vibration  co- 
incides with  the  period  of  the  vocal  chords  of  the  singer. 
In  each  case,  there  is  an  accumulation  of  the  effect,  simi- 
lar to  that  observed  when  you  stand  upon  a  plank-bridge, 
and  time  your  impulses  to  its  rate  of  vibration.  In  the 
case  of  the  singing  flame  already  referred  to,  you  had  the 
influence  of  period  exemplified  in  a  very  striking  manner. 


ACCOKD  AND  DISCOED  OF  VIBEATING  PEEIODS.         437 

It  responded  to  the  voice,  only  when  the  pitch  of  the  voice 
corresponded  to  its  own.  A  higher  and  a  lower  note  were 
equally  ineffective  to  put  the  flame  in  motion. 

I  have  shown  you  the  transparency  of  lampblack,  and 
the  far  more  wonderful  transparency  of  iodine,  to  the 
purely  thermal  rays ;  and  we  have  now  to  enquire  why 
iodine  stops  light  and  allows  heat  to  pass.  The  sole^dif- 
ference  between  light  and  radiant  heat  is  one  of  period. 
The  waves  of  the  one  are  short  and  of  rapid  recurrence, 
while  those  of  the  other  are  long,  and  of  slow  recurrence. 
The  former  are  intercepted  by  the  iodine,  and  the  latter 
are  allowed  to  pass.  Why?  There  can,  I  think,  be  only 
one  answer  to  this  question — that  the  intercepted  waves 
are  those  whose  periods  coincide  with  the  periods  of  os- 
cillation possible  to  the  atoms  of  the  dissolved  iodine. 
The  waves  transfer  their  motion  to  the  molecules  which 
synchronise  with  them.  Supposing  waves  of  any  period 
to  impinge  upon  an  assemblage  of  molecules  of  any  other 
period,  it  is,  I  think,  physically  certain  that  a  tremor  of 
greater  or  less  intensity  will  be  set  up  among  the  mole- 
cules ;  but  for  the  motion  to  accumulate,  so  as  to  produce 
sensible  absorption,  coincidence  of  period  is  necessary. 
Briefly  defined,  therefore,  transparency  is  synonymous 
with  discord,  while  opacity  is  synonymous  with  accord, 
between  the  periods  of  the  waves  of  ether  and  those  of 
the  molecules  of  the  body  on  which  they  impinge.  The 
opacity,  then,  of  our  solution  of  iodine  to  light  shows  that 
its  atoms  are  competent  to  vibrate  in  all  periods  which  lie 
within  the  limits  of  the  visible  spectrum ;  while  its  trans- 
parency to  the  extra-red  undulations  demonstrates  the  in- 
competency  of  its  atoms  to  vibrate  in  unison  with  the 
longer  waves. 

The  term  c  quality,'  as  applied  to  radiant  heat,  has 
been  already  defined;  the  ordinary  test  of  quality  being 
the  power  of  radiant  heat  to  pass  through  diathermic 


438  LECTURE  xn. 

"bodies.  If  the  heat  of  two  beams  be  transmitted  by  the 
selfsame  substance  in  different  proportions,  the  two  beams 
are  said  to  be  of  different  qualities.  Strictly  speaking, 
this  question  of  quality  is  one  of  period;  and  if  the  heat 
of  one  source  be  more  or  less  copiously  transmitted  than 
the  heat  of  another  source,  it  is  because  the  waves  of  ether 
excited  by  the  one  are  different  in  length  and  period  from 
those  excited  by  the  other.  When  we  raise  the  tempera- 
ture of  our  platinum  spiral,  we  alter  the  quality  of  its 
heat.  As  the  temperature  is  raised,  shorter  and  ever 
shorter  waves  mingle  in  the  radiation.  Dr.  Draper,  in  a 
very  beautiful  investigation,  has  shown  that  when  plati- 
num first  appears  luminous,  it  emits  only  red  rays ;  but 
as  its  temperature  augments,  orange,  yellow,  and  green 
are  successively  added  to  the  radiation;  and  when  the 
platinum  is  so  intensely  heated  as  to  emit  white  light,  the 
decomposition  of  that  light  gives  all  the  colours  of  the 
solar  spectrum. 

Almost  all  the  vapours  which  we  have  hitherto  exam- 
ined are  transparent  to  light,  while  all  of  them  are,  in 
some  degree,  opaque  to  obscure  rays.  This  proves  the 
incompetence  of  the  molecules  of  these  vapours  to  vibrate 
in  visual  periods,  and  their  competence  to  vibrate  in  the 
slower  periods  of  the  waves  which  fall  beyond  the  red  of 
the  spectrum.  Conceive,  then,  our  platinum  spiral  to  be 
gradually  raised  from  a  state  of  obscure  to  a  state  of  lu- 
minous heat ;  the  change  would  manifestly  tend  to  pro- 
duce discord  between  the  radiating  platinum  and  the 
molecules  of  our  vapours.  And  the  higher  we  raise  the 
temperature  of  our  platinum,  the  more  decided  will  be  the 
discord.  On  d  priori  grounds,  then,  we  should  infer,  that 
the  raising  of  the  temperature  of  the  platinum  spiral 
ought  to  augment  the  power  of  its  rays  to  pass  through 
our  list  of  vapours.  This  conclusion  is  entirely  verified 
by  the  experiments  recorded  in  the  following  tables  : 


INFLUENCE  OF  TEMPERATURE  ON  TRANSMISSION.      439 

RADIATION  THROUGH  VAPOURS.    SOURCE  OP  HEAT  :  PLATINUM  SPIRAL 
BARELY  VISIBLE  IN  THE  DARE!. 

Name  of  Vapour  Absorption  per  cent. 

Bisulphide  of  carbon  .  .  .      6'5 

Chloroform        .  .  .  .9-1 

Iodide  of  methyl  .  .  .12*5 

Iodide  of  ethyl  .  ,  .     21'0 

Benzol  .....     25'4 

Amylene  .  .  .     35'8 

Sulphuric  ether  .  .  .    43'4 

Formic  ether     .  .  .  .45*2 

Acetic  ether  .  ...    49'6 

With  the  same  platinum  spiral  raised  to  a  white  heat, 
the  following  results  were  obtained : — 

RADIATION  THKOUGH  VAPOURS.    SOURCE  OP  HEAT  :  WHITE-HOT 

PLATINUM  SPIRAL. 

Name  of  Yapour  Absorption  per  cent 

Bisulphide  of  carbon     .  .  .2-9 

Chloroform        .  .  .  .       6'6 

Iodide  of  methyl  .  .  .       7'8 

Iodide  of  ethyl  .  .  .     12-8 

Benzol  .....     16-5 
Amylene  .  .  .  .     2  2 '6 

Formic  ether     ....     25-1 
Sulphuric  ether  .  .  .     25 '9 

Acetic  ether      ....    2Y'2 

With  the  same  spiral,  brought  still  nearer  to  its  point 
of  fusion,  the  following  results  were  obtained  with  four  of 
the  vapours : — 

RADIATION  THROUGH  VAPOURS.  SOURCE  :  PLATINUM  SPIRAL  AT  AN 

INTENSE  WHITE  HEAT. 

Name  of  Vapour  Absorption 

Bisulphide  of  carbon  .  .  .2-5 

Chloroform        .  .  .  .       3'9 

Formic  ether     .  .  .  .     21 '3 

Sulphuric  ether  .  .  .    23 '7 


44:0  LECTURE   XII. 

Placing  the  results  obtained  with  the  respective  sources 
side  by  side,  the  influence  of  temperature  on  the  trans- 
mission comes  out  in  a  very  decided  manner : — 

ABSORPTION  OF  HEAT  BY  VAPOURS. 
Name  of  Vapour  Source  :  Platinum  Spiral 


Barely  visible    Bright  red 

White-hot 

Near  fusion 

Bisulphide  of  carbon   . 

6-5 

4-7 

2-9 

2-5 

Chloroform 

9-1 

6-3 

5-6 

3-9 

Iodide  of  methyl 

12-5 

9-6 

7-8 

Iodide  of  ethyl    . 

21-3 

17-7 

12-8 

Benzol 

26-4 

20-6 

16-5 

Amylene     . 

35-8 

27-5 

22-7 

Sulphuric  ether   . 

43-4 

31-4 

25-9 

23-7 

Formic  ether 

45-2 

31-9 

25-1 

21-3 

Acetic  ether 

49-6 

34-6 

27-2 

The  gradual  augmentation  of  penetrative  power,  as  the 
temperature  is  augmented,  is  here  very  manifest.  By 
raising  the  spiral  from  a  barely  visible  to  an  intense  white 
heat,  we  reduce  the  absorption,  in  the  case  of  bisulphide 
of  carbon  and  chloroform,  to  less  than  one-half.  At  barely 
visible  redness,  moreover,  56*6  and  54*8  per  cent,  pass 
through  sulphuric  and  formic  ether  respectively ;  while  of 
the  intensely  white-hot  spiral,  76 '3  and  78'7  per  cent,  pass 
through  the  same  vapours.*  Thus,  by  augmenting  the 
temperature  of  the  solid  platinum,  we  introduce  into  the 
radiation  waves  of  shorter  period,  which,  being  in  discord 
with  the  periods  of  the  vapours,  pass  more  easily  through 
them. 

Running  the  eye  along  the  numbers  which  express  the 
absorptions  of  sulphuric  and  formic  ether  in  the  last  table, 
we  find  that,  for  the  lowest  heat,  the  absorption  of  the 
latter  exceeds  that  of  the  former ;  for  a  bright  red  heat* 
they  are  nearly  equal,  but  the  formic  still  retains  a  slight 
predominance;  at  a  white  heat,  however,  the  sulphuric 

*  The  transmission  is  found  by  subtracting  the  absorption  from  100. 


PERIODS   OF   FORMIC   AND   SULPHURIC   ETHER.        441 

slips  in  advance,  and  at  the  heat  near  fusion  its  predomi- 
nance is  decided.  I  have  tested  this  result  in  various 
ways,  and  by  multiplied  experiments,  and  placed  it  be- 
yond doubt.  We  may  at  once  infer  from  it  that  the  ca- 
pacity of  the  molecule  of  formic  ether  to  enter  into  rapid 
vibration  is  less  than  that  of  sulphuric,  and  thus  we  ob- 
tain a  glimpse  of  the  inner  character  of  these  bodies.  By 
augmenting  the  temperature  of  the  spiral,  we  produce 
vibrations  of  quicker  periods,  and  the  more  of  these  that 
are  introduced,  the  more  opaque,  in  comparison  with 
formic  ether,  does  sulphuric  ether  become.  The  atom  of 
oxygen  which  formic  ether  possesses,  in  excess  of  sul- 
phuric, renders  it  more  sluggish  as  a  vibrator.  Experi- 
ments made  with  a  source  of  100°  C.,  establish  more  de- 
cidedly the  preponderance  of  the  formic  ether  for  vibra- 
tions of  slow  period. 

RADIATION  THROUGH  VAPOURS.    SOURCE  :  LESLIE'S  CUBE,  COATED  WITH 

LAMPBLACK.     TEMPERATURE,  212°  FAHR. 
Name  of  Vapour  Absorption  per  cent. 

Bisulphide  of  carbon  .  6'6 

Iodide  of  methyl        .  .  .       18'8 

Chloroform    .  .  .  ,21'6 

Iodide  of  ethyl          .  .  .       29'0 

Benzol  .  34-5 

Amylene        ....      47*1 
Sulphuric  ether         .  .  .       64'1 

Formic  ether  .  .  .       60'4 

Acetic  ether  .  .  .  .  '     6 9 -9 

For  heat  issuing  from  this  source,  the  absorption  by  formic 
ether  is  6 '3  per  cent,  in  excess  of  that  by  sulphuric. 

But  in  this  table  we  notice  another  case  of  reversal. 
In  all  the  experiments  with  the  platinum  spiral  thus  far 
recorded,  chloroform  showed  itself  less  energetic,  as  an 
absorber,  than  iodide  of  methyl;  but  here  chloroform 
shows  itself  to  be  decidedly  the  more  powerful  of  the  two. 
This  result  has  been  placed  beyond  doubt,  by  repeated 
1 J 


LECTURE   XII. 

experiments.  To  the  radiation  emitted  by  lampblack, 
heated  to  212°,  chloroform  is  certainly  more  opaque  than 
iodide  of  methyl. 

We  have  hitherto  occupied  ourselves  with  the  radia- 
tion from  heated  solids :  I  will  now  pass  on  to  the  exami- 
nation of  the  radiation  from  flames.  The  first  experi- 
ments were  made  with  a  steady  jet  of  gas,  issuing  from  a 
small  circular  burner,  the  flame  being  long  and  tapering. 
The  top  and  bottom  of  the  flame  were  excluded,  and  its 
most  brilliant  portion  was  chosen  as  the  source.  The  fol- 
lowing results  were  obtained :  . 

RADIATION  OF  HEAT  THROUGH  VAPOURS.    SOURCE  :  A  HIGHLY  LUMINOUS 

JET  OF  GAS. 

Name  of  Vapour                    Absorption  White-hot  Spiral 

Bisulphide  of  carbon          .       9-8  2*9 

Chloroform        .         .         .     12'0  5'6 

Iodide  of  methyl                .     16-5  7 "8 

Iodide  of  ethyl                   .19-5  12 -8 

Benzol       ....     22-0  16'5 

Amylene            .        .        .     30'2  22'7 

Formic  ether     .         .         .     34'6  25'9 

Sulphuric  ether          .        .     35-7  25-1 

Acetic  ether      .        .        .     38'7  27'2 

It  is  interesting  to  compare  the  heat  emitted  by  the 
white-hot  carbon  with  that  emitted  by  the  white-hot  plati- 
num ;  and  to  facilitate  the  comparison,  I  have  placed  be- 
side the  results  given  in  the  last  table  those  recorded  in  a 
former  one.  The  emission  from  the  flame  is  thus  proved 
to  be  far  more  powerfully  absorbed  than  the  emission  from 
the  spiral.  Doubtless,  however,  the  carbon,  in  reaching 
incandescence,  passes  through  lower  stages  of  temperature, 
and  in  those  stages  emits  heat  more  in  accord  with  our 
vapours.  It  is  also  mixed  with  the  vapour  of  water  and 
carbonic  acid,  both  of  which  contribute  their  quota  to  the 
total  radiation.  It  is  therefore  probable  that  the  greater 


RADIATION  FKOM  FLAMES.  443 

absorption  of  the  heat  emitted  by  the  flame  is  due  to  the 
slower  periods  of  the  substances,  which  are  unavoidably 
mixed  with  the  white-hot  carbon  to  which  the  flame  mainly 
owes  its  light. 

The  next  source  of  heat  employed  was  the  flame  of  a 
Bunsen's  burner,*  the  temperature  of  which  is  known  to 
be  very  high.  The  flame  was  of  a  pale-blue  colour,  and 
emitted  a  very  feeble  light.  The  following  results  were 
obtained : — - 

RADIATION  OP  HEAT  THROUGH  VAPOURS.    SOURCE  :  PALE-BLUE  FLAME  OP 

BUNSEN'S  BURNER. 
Name  of  Vapour  Absorption 

Chloroform 6'2 

Bisulphide  of  carbon    .        .        .        .     ll'l 

Iodide  of  ethyl 14-0 

Benzol 17'9 

Amylene 24'2 

Sulphuric  ether 31'9 

Formic  ether        .  33'3 

Acetic  ether 36'3 

The  total  heat  radiated  from  the  flame  of  Bunsen's 
burner  is  much  less  than  that  radiated  when  the  incandes- 
cent carbon  is  present  in  the  flame.  The  moment  the  air 
is  permitted  to  mix  with  the  luminous  flame,  the  radiation 
falls  so  considerably,  that  the  diminution  is  at  once  de- 
tected, even  by  the  hand  or  face  brought  near  the  flame. 
Comparing  the  two  last  tables,  we  see  that  the  radiation 
from  the  Bunsen's  burner  is,  on  the  whole,  less  powerfully 
absorbed  than  that  from  the  luminous  gas  jet.  In  some 
cases,  as  in  that  of  formic  ether,  they  come  very  close  to 
each  other;  in  the  case  of  amylene,  and  a  few  other  sub- 
stances, they  differ  more  markedly.  But  an  extremely 
interesting  case  of  reversal  here  shows  itself.  -Bisulphide 
of  carbon,  instead  of  being  first,  stands  decidedly  below 

*  Described  in  Lecture  II. 


4:4:4:  LECTUKE   XII. 

chloroform.  With  the  luminous  jet,  the  absorption  of  bi- 
sulphide of  carbon  is  to  that  of  chloroform  as  100:122, 
while  with  the  flame  of  Bunsen's  burner  the  ratio  is 
100:56;  the  removal  of  the  lampblack  from  the  flame 
more  than  doubles  the  relative  transparency  of  the  chloro- 
form. We  have  here,  moreover,  another  instance  of  the 
reversal  of  formic  and  sulphuric  ether.  For  the  luminous 
jet,  the  sulphuric  ether  is  decidedly  the  more  opaque ;  for 
the  flame  of  Bunsen's  burner,  it  is  excelled  in  opacity  by 
the  formic. 

The  main  radiating  bodies  in  the  flame  of  a  Bunsen's 
burner,  are,  no  doubt,  aqueous  vapour  and  carbonic  acid. 
Highly  heated  nitrogen  is  also  present,  which  may  pro- 
duce a  sensible  effect.  But  the  main  source  of  the  radia- 
tion is,  no  doubt,  the  aqueous  vapour  and  the  carbonic 
acid.  I  wished  to  separate  these  two  constituents,  and  to 
study  them  separately.  The  radiation  of  aqueous  vapour 
could  be  obtained  from  a  flame  of  pure  hydrogen,  while 
that  of  carbonic  acid  could  be  obtained  from  an  ignited  jet 
of  carbonic  oxide.  To  me  the  radiation  from  the  hydro- 
gen flame  possessed  a  peculiar  interest ;  for  notwithstand- 
ing the  high  temperature  of  such  a  flame,  I  thought  it 
likely  that  the  accord  between  its  periods  of  vibration  and 
those  of  the  cool  aqueous  vapour  of  the  atmosphere  would 
Btill  be  such  as  to  cause  the  atmospheric  vapour  to  exert  a 
special  absorbent  power  upon  the  radiation.  The  follow- 
ing experiments  test  this  surmise : — 

RADIATION  THROUGH  ATMOSPHERIC  AIR.    SOURCE  :  A  HYDROGEN  FLAME. 

Absorption 

Dry  air 0 

Undried  air 17'2 

Thus,  in  a  polished  tube  4  feet  long,  the  aqueous  vapour 
of  our  laboratory  air  absorbed  17  per  cent,  of  the  radiation 
from  the  hydrogen  flame.  A  platinum  spiral,  raised  by 


KADIATION   FROM   HYDKOGEN   JTf&KE-  445 


electricity  to  a  degree  of  incandescence  noV^gceater  than 
that  obtainable  by  plunging  a  wire  into  the  hydrogen 
flame,  being  used  as  a  source  of  heat,  the  undried  air  of 
the  laboratory  was  found  to  absorb 

5*8  per  cent. 

of  its  radiation,  or  one-third  of  the  quantity  absorbed  in 
the  case  of  the  flame  of  hydrogen. 

The  plunging  of  a  spiral  of  platinum  wire  into  the 
flame  reduces  its  temperature ;  but  at  the  same  time  intro- 
duces vibrations,  which  are  not  in  accord  with  those  of 
aqueous  vapour ;  the  absorption,  by  ordinary  undried  air, 
of  heat  emitted  by  this  composite  source  amounted  to 

8'6  per  cent. 

On  humid  days,  the  absorption  of  the  rays  emitted  by  a 
hydrogen  flame  exceeds  even  the  above  large  figure.  Em- 
ploying the  same  experimental  tube  and  a  new  burner,  the 
experiments  were  repeated  some  days  subsequently,  with 
the  following  result : — 

KADIATION  THROUGH  AIR.    SOURCE  :  HYDROGEN  FLAME. 

Absorption 
Dry  air        ......       0 

Undried  air         .        .         .        .         .20-3 

The  physical  causes  of  transparency  and  opacity  have 
been  already  pointed  out;  and  we  may  infer  from  the 
foregoing  powerful  action  of  atmospheric  vapour  on  the 
radiation  from  the  hydrogen  flame,  that  accord  reigns  be- 
tween the  oscillating  molecules  of  the  flame  at  a  temper- 
ature of  5898°  Fahr.,  and  the  molecules  of  aqueous  vapour 
at  a  temperature  of  60°  Fahr.  The  enormous  temperature 
of  the  hydrogen  flame  increases  the  amplitude  but  does 
not  change  the  rate  of  oscillation. 

We  must  devote  a  moment's  attention,  in  passing,  to 
the  word  { amplitude '  here  employed.  The  pitch  of  a  note 


446  LECTUEE   XH. 

depends  solely  on  the  number  of  aerial  waves  which 
strike  the  ear  in  a  second.  The  loudness,  or  intensity,  of 
a  note  does  not  at  all  depend  upon  the  rapidity  with 
which  the  waves  follow  each  other,  but  on  the  distance 
within  which  the  separate  atoms  of  air  vibrate.  This  dis- 
tance is  called  the  amplitude  of  the  vibration.  When  we 
pull  a  harp-string  very  gently  aside,  and  let  it  go,  it  dis- 
turbs the  air  but  little ;  the  amplitude  of  the  vibrating 
air-atoms  is  small,  and  the  intensity  of  the  sound  feeble. 
But  if  we  pull  the  string  vigorously  aside,  on  letting  it 
go,  we  have  a  note  of  the  same  pitch  as  before,  but,  as  the 
amplitude  of  vibration  is  greater,  the  sound  is 'more  in- 
tense. While,  then,  the  wave-length,  or  period  of  recur- 
rence, is  independent  of  the  amplitude,  it  is  this  latter 
which  determines  the  loudness  of  the  sound. 

The  same  holds  good  for  light  and  radiant  heat.  Here 
the  individual  ether  particles  vibrate,  to  and  fro  across  the 
line  of  propagation ;  and  the  extent  of  their  excursion  is 
called  the  amplitude  of  the  vibration.  We  may,  as  in  the 
case  of  sound,  have  the  same  wave-length  with  very  dif- 
ferent amplitudes,  or,  as  in  the  case  of  water,  we  may 
have  high  waves  and  low  waves,  with  the  same  distance 
between  crest  and  crest.  Now,  while  the  colour  of  light, 
and  the  quality  of  radiant  heat,  depend  entirely  upon  the 
length  of  the  ethereal  waves,  the  intensity  of  the  light 
and  heat  is  determined  by  the  amplitude.  And,  inasmuch 
as  it  has  been  shown,  that  the  periods  of  vibration  of  a 
hydrogen  flame  coincide  with  those  of  cool  aqueous  va- 
pour, we  are  compelled  to  conclude  that  the  enormous 
temperature  of  the  flame  is  not  due  to  the  rapidity,  but  to 
the  extraordinary  amplitude  of  its  molecular  vibration. 

The  other  component  of  the  flame  of  Buusen's  burner 
is  carbonic  acid,  and  the  radiation  of  this  substance  is  im- 
mediately obtained  from  a  flame  of  carbonic  oxide.  Of 
the  radiation  from  this  source,  the  small  amount  of  car- 


RADIATION   FKOH   CAEBONIG   OXIDE   FLAME.  447 

bonic  acid  diffused  in  the  air  of  our  laboratory  absorbed 
13-8  per  cent.  This  high  absorption  proves  that  the  vibra- 
tions of  the  molecules  of  carbonic  acid,  within  the  flame, 
are  synchronous  with  the  vibrations  of  those  of  the  car- 
bonic acid  of  the  atmosphere.  The  temperature  of  the 
flame,  however,  is  5508°  Fahr.,  while  that  of  the  atmos- 
phere is  only  60°.  But  if  the  high  temperature  is  incom- 
petent to  change  the  rate  of  oscillation,  we  may  expect 
carbonic  acid,  when  used  in  large  quantities,  to  be  highly 
opaque  to  the  radiation  from  the  carbonic  oxide  flame. 
Here  follow  the  results  of  experiments  executed  to  test 
this  conclusion : — 


RADIATION  THROUGH  DRY  CARBONIC  ACID. 

SOURCE:  CARBONIC 

OXIDE  FLAME. 

Pressure  in  inches 

Absorption 

1-0 

48-0 

2-0 

55-5 

3-0 

60-3 

4-0 

65-1 

5-0 

68-6 

10-0 

74-3 

For  frne  rays  emanating  from  the  heated  solids  employed 
in  our  former  researches,  carbonic  acid  proved  to  be  one 
of  the  most  feeble  absorbers ;  but  here,  when  the  waves 
sent  into  it  emanate  from  molecules  of  its  own  substance, 
its  absorbent  energy  is  enormous.  The  thirtieth  of  an 
atmosphere  of  the  gas  cuts  off  half  the  entire  radiation ; 
while  at  a  pressure  of  4  inches,  65  per  cent,  of  the  radia- 
tion is  intercepted. 

The  energy  of  olefiant  gas,  both  as  an  absorbent  and 
a  radiant,  is  now  well  known.  For  the  solid  sources  of 
heat  just  referred  to,  its  power  is  incomparably  greater 
than  that  of  carbonic  acid ;  but  for  the  radiation  from  the 
carbonic  oxide  flame,  the  power  of  olefiant  gas  is  feeble, 
when  compared  with  that  of  carbonic  acid.  This  is  proved 
by  the  experiments  recorded  in  the  following  table : — 


448  LECTUKE  xn. 

RADIATION  THROUGH  DRY  OLEFIANT  GAS.  SOURCE  :  CARBONIC 

OXIDE  FLAME. 

Pressure  in  inches                   Absorption  From  last  Table 

1-0                                   23-2  48-0 

2-0                                  32-7  55-5 

3-0                                  44-0  60-3 

4-0                                  50-6  65-1 

5-0                                  55-1  68-6 

lO'O                                  65-5  74-3     . 

Beside  the  absorption  by  olefiant  gas,  I  have  placed 
that  by  carbonic  acid  derived  from  the  last  table.  The 
superior  power  of  the  acid  is  most  decided  in  the  smaller 
pressures ;  at  a  pTessure  of  an  inch  it  is  twice  that  of  the 
olefiant  gas.  The  substances  approach  each  other  more 
closely,  as  the  quantity  of  gas  augments.  Here,  in  fact, 
both  of  them  approach  perfect  opacity,  and  as  they  draw 
near  to  this  common  limit,  their  absorptions,  as  a  matter 
of  course,  approximate. 

These  experiments  prove  that  the  presence  of  an  infin- 
itesimal quantity  of  carbonic  acid  gas  might  be  detected, 
by  its  action  on  the  rays  emitted  by  a  carbonic  oxide 
flame.  The  action,  for  example,  of  the  carbonic  acid  ex- 
pired by  the  lungs  is  very  decided.  An  india-rubber  bag 
was  filled  from  the  lungs ;  it  contained,  therefore,  both  the 
aqueous  vapour  and  the  carbonic  acid  of  the  breath.  The 
air  from  the  bag  was  then  conducted  through  a  drying 
apparatus,  the  moisture  being  thus  removed,  and  the 
neutral  air  and  active  carbonic  acid  permitted  to  enter 
the  experimental  tube.  The  following  results  were  ob- 
tained : — 

AIR  FROM  THE  LUNGS  CONTAINING -C  02.  SOURCE  :  CARBONIC  OXIDE 

FLAME. 

Pressure  in  inches  Absorption 

1  12-0 

3  25-0 

5  33-3 

30  50-0 


PHYSICAL  ANALYSIS   OF   HUMAN  BREATH.  449 

Thus,  the  tube  filled  with  the  dry  exhalation  from  the 
lungs  intercepted  50  per  cent,  of  the  entire  radiation  from 
a  carbonic  oxide  flame.  It  is  quite  manifest  that  we  have 
here  a  means  of  testing,  with  surpassing  delicacy,  the 
amount  of  carbonic  acid  emitted  under  various  circum- 
stances from  the  lungs. 

The  application  of  radiant  heat  to  the  determination 
of  the  carbonic  acid  of  the  breath  has  been  illustrated,  by 
a  series  of  experiments,  executed  under  my  direction  by 
my  assistant,  Mr.  Barrett.  The  deflection  produced  by 
the  breath,  freed  from  its  moisture,  but  retaining  its  car- 
bonic acid,  was  first  determined.  Carbonic  acid,  artifi- 
cially prepared,  was  then  mixed  with  perfectly  dry  air,  in 
such  proportions  that  its  action  upon  the  radiant  heat  was 
the  same  as  that  of  the  carbonic  acid  of  the  breath.  The 
percentage  of  the  former  being  known,  immediately  gave 
that  of  the  latter.  I  here  give  the  results  of  three  chemi- 
cal analyses,  determined  by  Dr.  Frankland,  as  compared 
with  three  physical  analyses  performed  by  my  assistant : — 

PERCENTAGE  OF  CARBONIC  ACID  IN  HUMAN  BREATH. 

By  chemical  analysis  By  chemical  analysis 

4-311  4-00 

4-66  4-56 

5-33  6-22 

The  agreement  between  the  results  is  very  fair.  Doubt- 
less, with  greater  practice  a  closer  agreement  will  be  at- 
tained. We  shall  thus  find,  in  the  quantity  of  ethereal 
motion  which  it  is  competent  to  destroy,  an  accurate  and 
practical  measure  for  the  amount  of  carbonic  acid  expired 
from  the  human  lungs. 

Water  at  moderate  thickness  is  a  very  transparent  sub- 
stance ;  that  is  to  say,  the  periods  of  its  molecules  are  in 
discord  with  those  of  the  visible  spectrum.  It  is  also 
highly  transparent  to  the  extra-violet  rays ;  so  that  we 
may  safely  infer  from  the  deportment  of  this  substance, 
its  incompetence  to  enter  into  rapid  molecular  vibration. 


450  LECTUKE   XH. 

When,  however,  we  once  quit  the  visible  spectrum  for  the 
rays  beyond  the  red,  the  opacity  of.  the  substance  begins 
to  show  itself;  for  such  rays,  indeed,  its  absorbent  power 
is  unequalled.  The  synchronism  of  the  periods  of  the 
water  molecules  with  those  of  the  extra-red  waves  is  thus 
demonstrated.  We  have  already  seen  that  undried  at- 
mospheric air  manifests  an  extraordinary  opacity  for  the 
radiation  from  a  hydrogen  flame,  and  from  this  deport- 
ment we  inferred  the  synchronism  of  the  cold  vapour  of 
the  air,  and  the  hot  vapour  of  the  flame.  But  if  the  pe- 
riods of  a  vapour  be  the  same  as  those  of  its  liquid,  we 
ought  to  find  water  highly  opaque  to  the  radiation  from  a 
hydrogen  flame.  Here  are  the  results  obtained  with  five 
different  thicknesses  of  the  liquid : — 

RADIATION  THROUGH  WATER.     SOURCE  :  HYDROGEN  FLAME. 

Thickness  of  liquid 


0-02  inch     0-04  inch      0'07  inch       0'14  inch       0'27  inch 
Transmission  per  cent.     6'8  2*8  I'l  0'5  0*0 

Through  a  layer  of  water  0*36  of  an  inch  thick,  Mel- 
loni  found  a  transmission  of  11  per  cent,  of  the  heat  of  an 
Argand  lamp.  Here  we  employ  a  source  of  higher  tem- 
perature, and  a  layer  of  water  only  0*27  of  an  inch,  and 
find  the  whole  of  the  heat  intercepted.  A  layer  of  water 
0'27  of  an  inch  in  thickness  is  perfectly  opaque  to  the  ra- 
diation from  a  hydrogen  flame,  while  a  layer  about  one- 
tenth  of  the  thickness  employed  by  Melloni,  cuts  off  more 
than  97  per  cent,  of  the  entire  radiation.  Hence,  we  may 
infer  the  coincidence  in  period  between  cold  water  and 
aqueous  vapour  heated  to  a  temperature  of  5898°  Fahr. 
(3259°  C.) 

From  the  opacity  of  water  to  the  radiation  from 
aqueous  vapour,  we  may  infer  the  opacity  of  aqueous  va- 
pour to  the  radiation  from  water,  and  hence  conclude  that 
the  very  act  of  nocturnal  refrigeration  which  causes  the 
condensation  of  water  on  the  earth's  surface,  gives  to  ter- 


INFLUENCE   OF   PLANETARY  ATMOSPHEEES.  451 

restrial  radiation  that  particular  character  which  renders 
it  most  liable  to  be  intercepted  by  our  atmosphere,  and 
thus  prevented  from  wasting  itself  in  space. 

This  is  a  point  which  deserves  a  moment's  further  con- 
sideration. I  find  that  olefiant  gas  contained  in  a  polished 
tube  4  feet  long,  absorbs  about  80  per  cent,  of  the  radia- 
tion from  an  obscure  source.  A  layer  of  the  same  gas  2 
inches  thick  absorbs  33  per  cent.,  a  layer  1  inch  thick  ab- 
sorbs 26  per  cent.,  while  a  layer  -rJ-g-th  of  an  inch  in  thick- 
ness absorbs  2  per  cent,  of  the  radiation.  Thus  the  ab- 
sorption increases,  and  the  quantity  transmitted  dimin- 
ishes, as  the  thickness  of  the  gaseous  layer  is  augmented. 
Let  us  .now  consider  for  a  moment  the  effect  upon  the 
earth's  temperature  of  a  shell  of  olefiant  gas,  surrounding 
our  planet  at  a  little  distance  above  its  surface.  The  gas 
would  be  transparent  to  the  solar  rays,  allowing  them, 
without  sensible  hindrance,  to  reach  the  earth.  HerCj 
however,  the  luminous  heat  of  the  sun  would  be  converted 
into  non-luminous  terrestrial  heat ;  at  least  26  per  cent,  of 
this  heat  would  be  intercepted  by  a  layer  of  gas  one  inch 
thick,  and  in  great  part  returned  to  the  earth.  Under 
such  a  canopy,  trifling  as  it  may  appear,  and  perfectly 
transparent  to  the  eye,  the  earth's  surface  would  be  main- 
tained at  a  stifling  temperature. 

A  few  years  ago,  a  work  possessing  great  charms  of 
style  and  ingenuity  of  reasoning,  was  written  to  prove 
that  the  more  distant  planets  of  our  system  are  uninhab- 
itable. Applying  the  law  of  inverse  squares  to  their  dis- 
tances from  the  sun,  the  diminution  of  temperature  was 
found  to  be  so  great,  as  to  preclude  the  possibility  of 
human  life  in  the  more  remote  members  of  the  solar  sys- 
tem. But  in  those  calculations  the  influence  of  an  atmos- 
pheric envelope  was  overlooked,  and  this  omission  vitiated 
the  entire  argument.  It  is  perfectly  possible  to  find  an 
atmosphere  which  would  act  the  part  of  a  barb  to  the 
solar  rays,  permitting  their  entrance  towards  the  planet, 


452  LECTURE   XII. 

but  preventing  their  withdrawal.  For  example,  a  layer 
of  air  two  inches  in  thickness,  and  saturated  with  the 
vapour  of  sulphuric  ether,  would  offer  very  little  resist- 
ance to  the  passage  of  the  solar  rays,  but  I  find  that  it 
would  cut  off  fully  35  per  cent,  of  the  planetary  radiation. 
It  would  require  no  inordinate  thickening  of  the  layer  of 
vapour  to  double  this  absorption ;  and  it  is  perfectly  evi- 
dent that,  with  a  protecting  envelope  of  this  kind,  per- 
mitting the  heat  to  enter,  but  preventing  its  escape,  a 
comfortable  temperature  might  be  obtained  on  the  surface 
of  our  most  distant  planet. 

Dr.  Akin  was  the  first  to  maintain  the  opinion,  which 
I  hold  to  be  correct,  that  the  vibrating  periods  of  a  hy- 
drogen flame  must  be  extra  red ;  and  that  consequently, 
when  a  platinum  wire  is  plunged  into  a  hydrogen  flame 
and  rendered  white-hot,  its  oscillating  periods  must  be  dif- 
ferent from  those  of  the  flame  to  which  it  owes  its  incan- 
descence. We  have,  in  this  case,  a  conversion  of  unvisual 
periods  into  visual  ones.  This  shortening  of  the  periods 
must  augment  the  discord  between  the  radiating  source 
and  our  series  of  liquids,  whose  periods  are  long,  and 
hence  augment  their  transparency  to  the  radiation.  This 
conclusion  is  verified  by  the  following  experiments : — 

RADIATION  THROUGH  LIQUIDS.     SOURCES:  1.  HYDROGEN  FLAME ;  2. 
HYDROGEN  FLAME  AND  PLATINUM  SPIRAL. 
Transmission 

,,  fT     .,   Thickness  of  liquid  O04  inch:     Thickness  of  liquid  0-07  inch : 

Name  of  Liquid  -plame  only_  plame  aml  gpiral>     Flame  on]y^  Elame  and  Bpiral> 

Bisulphide  of  carbon  77'7  87'2  70-4  86'0 

Chloroform     .  .  54'0  72'8  60'7  69'0 

Iodide  of  methyl  .  31'6  42-4  26'2  36-2 

Iodide  of  ethyl  .  30'3  36-8  24'2  32-6 

Benzol   .        .  .  24'1  32'6  17'9  28'8 

Amylene         .  .  14'9  25'8  12'4  24'3 

Sulphuric  ether  .  13'1  22-6  8'1  22-0 

Acetic  ether  .  .  lO'l  18-3  6'6  18'5 

Alcohol          .  .  9-4  14-7  5'8  12-3 

Water    .  3'2  7'5  2*0  6'4 


SHORTENING  OF  VIBRATING   PERIOD.  453 

The  transmission  is  here  shown  to  be  considerably  aug- 
mented by  the  introduction  of  the  platinum  wire. 

And  here  we  find  ourselves  in  a  position  to  offer  solu- 
tions of  various  facts,  which  have  hitherto  stood  out  as 
enigmas  in  researches  upon  radiant  heat.  It  was  for  a 
time  generally  supposed  that  the  power  of  heat  to  pene- 
trate diathermic  substances  augmented  as  the  temperature 
of  the  source  became  more  elevated.  Knoblauch  con- 
tended against  this  notion,  showing  that  the  heat  emitted 
by  a  platinum  wire  plunged  in  an  alcohol  flame  was  less 
absorbed,  by  certain  diathermic  substances,  than  the  heat 
of  the  flame  itself,  and  justly  arguing  that  the  tempera- 
ture of  the  spiral  could  not  be  higher  than  that  of  the 
body  from  which  it  derived  its  heat.  A  plate  of  trans- 
parent glass  being  introduced  between  his  incandescent 
platinum  spiral  and  his  thermo-electric  pile,  the  deflection 
of  his  needle  fell  from  35°  to  19°;  while,  when  the  source 
was  the  flame  of  alcohol,  without  the  spiral,  the  deflection 
fell  from  35°  to  16°.  This  proved  the  radiation  from  the 
flame  to  be  intercepted  more  powerfully  than  that  from 
the  spiral ;  or,  in  other  words,  that  the  heat  emanating 
from  the  body  of  highest  temperature  possessed  the  least 
penetrative  power.  Melloni  afterwards  corroborated  this 
experiment. 

Transparent  glass  allows  the  rays  of  the  visible  spec- 
trum to  pass  freely  through  it ;  but  it  is  well  known  to  be 
highly  opaque  to  the  radiation  from  obscure  sources ;  or  to 
waves  of  long  period.  A  plate  O'l  of  an  inch  thick  inter- 
cepts all  the  rays  from  a  source  of  100°  C.,  and  transmits 
only  6  per  cent,  of  the  heat  emitted  by  copper  raised  to 
400°  C.  Now  the  products  of  an  alcohol  flame  are  aqueous 
vapour  and  carbonic  acid,  whose  waves  have  been  proved 
to  be  of  slow  period ;  of  the  particular  character,  conse- 
quently, most  powerfully  intercepted  by  glass.  But  by 
plunging  a  platinum  wire  into  such  a  flame,  we  virtually 


4:54:  LECTUEE  XH. 

convert  its  heat  into  heat  of  higher  refrangibilit j ;  we 
change  the  long  periods  into  shorter  ones,  and  thus  estab- 
lish the  discord  between  the  periods  of  the  source  and  the 
periods  of  the  diathermic  glass,  which,  as  before  defined, 
is  the  physical  cause  of  transparency.  On  purely  d  priori 
grounds,  therefore,  we  might  infer  that  the  introduction 
of  the  platinum  spiral  would  augment  the  penetrative 
power  of  the  heat.  With  a  plate  of  glass  Melloni,  in  fact, 
found  the  following  transmissions  for  the  flame  and  the 
spiral : — 

For  the  flame  For  the  platinum 

41-2  52-8 

The  same  remarks  apply  to  the  transparent  selenite  exam- 
ined by  Melloni.  This  substance  is  highly  opaque  to  the 
extra-red  undulations ;  but  the  radiation  from  an  alcohol 
flame  is  mainly  extra-red,  and  hence  the  opacity  of  the 
selenite  to  this  radiation.  The  introduction  of  the  plati- 
num spiral  shortens  the  periods  and  augments  the  trans- 
mission. Thus,  with  a  specimen  of  selenite,  Melloni  found 
the  transmissions  to  be  as  follows : — 

Flame  Platinum 

4-4  19-5 

So  far  the  results  of  Melloni  coincide  with  those  of  M. 
Knoblauch ;  but  the  Italian  philosopher  pursues  the  mat- 
ter further,  and  shows  that  M.  Knoblauch's  results,  though 
true  for  the  particular  substances  examined  by  him,  are 
not  true  of  diathermic  media  generally.  Melloni  shows 
that  in  the  case  of  black  glass  and  UacJt  mica,  a  striking 
inversion  of  the  effect  is  observed:  through  these  sub- 
stances the  radiation  from  the  flame  is  more  copiously 
transmitted  than  that  from  the  platinum.  For  black  glass 
he  found  the  following  transmissions : — 

From  the  flame  From  the  platinum 

52-6  42-8 


RESULTS  OF  MELLONI  AND  KNOBLAUCH  EXPLAINED.    455 

And  for  a  plate  of  black  mica  the  following  transmis- 
sions : — 

From  the  flame  From  the  platinum 

62-8  52-5 

These  results  were  left  unexplained  by  Melloni,  but 
the  solution  is  now  easy.  The  black  glass  and  the  black 
mica  owe  their  blackness  to  the  carbon  incorporated  in 
them,  and  the  opacity  of  this  substance  to  light,  as  already 
remarked,  proves  the  accord  of  its  vibrating  periods  with 
those  of  the  visible  spectrum.  But  it  has  been  shown  that 
carbon  is,  in  a  considerable  degree,  pervious  to  the  waves 
of  long  period ;  that  is  to  say,  to  such  waves  as  are  emit- 
ted by  a  flame  of  alcohol.  The  case  of  the  carbon  is 
therefore  precisely  antithetical  to  that  of  the  transparent 
glass,  the  former  transmitting  the  heat  of  long  period,  and 
the  latter  that  of  short  period  most  freely.  Hence  it  fol- 
lows that  the  introduction  of  the  platinum  wire,  by  convert- 
ing the  long  periods  of  the  flame  into  short  ones,  augments 
the  transmission  through  the  transparent  glass  and  selenite, 
and  diminishes  it  through  the  opaque  glass  and  mica. 

NOTE. 

The  following  Appendix  contains  the  last  published  investigation  on 
the  visible  and  invisible  rays  emitted  by  various  bodies. 


APPENDIX  TO  LECTURE  XII. 


ON  LUMINOUS  AND  OBSCURE  RADIATION.* 

SIR  WILLIAM  HEESOHEL  discovered  the  obscure  rays  of  the  sun, 
and  proved  the  position  of  maximum  heat  to  be  beyond  the  red  of 
the  solar  spectrum.t  Forty  years  subsequently  Sir  John  Herschel 
succeeded  in  obtaining  a  thermograph  of  the  calorific  spectrum, 
and  in  giving  striking  visible  evidence  of  its  extension  beyond  the 
red.J  Melloni  proved  that  an  exceedingly  large  proportion  of  the 
emission  from  a  flame  of  oil,  of  alcohol,  and  from  incandescent 
platinum  heated  by  a  flame  of  alcohol,  is  obscure.§  Dr.  Akin  in- 
ferred from  the  paucity  of  luminous  rays,  as  evident  to  the  eye, 
and  a  like  paucity  of  extra-violet  rays,  as  proved  by  the  experi- 
ments of  Dr.  Miller,  that  the  radiation  from  a  flame  of  hydrogen 
must  be  mainly  extra-red ;  and  he  concluded  from  this  that  the 
glowing  of  a  platinum  wire  in  a  hydrogen  flame,  and  also  the 
brightness  of  the  Drummond  light  in  the  oxyhydrogen  flame,  were 
produced  by  a  change  in  the  period  of  vibration.  |  By  a  different 
mode  of  reasoning  I  arrived  at  the  same  conclusion  myself,  and 
published  the  conclusion  subsequently. IT 

A  direct  experimental  demonstration  of  the  character  of  the 
radiation  from  a  hydrogen  flame  was,  however,  wanting,  and  this 

*  From  the  Philosophical  Magazine  for  November  1864. 

f  Phil.  Trans.  1800. 

\  Phil.  Trans.  1840.  I  hope  very  soon  to  be  able  to  turn  my  atten- 
tion to  the  remarkable  results  described  in  Note  III.  of  Sir  J.  Herschel's 
paper. 

§  La  Thermochrose,  p.  304. 

|  Reports  of  the  British  Association,  1863. 

Tf  Phil.  Trans,  vol.  cliv.  p.  237. 


LUMINOUS   AND   OBSCUEE   RADIATION.  457 

want  I  have  sought  to  supply.  I  had  constructed  for  rno,  by  Mr. 
Becker,  lenses  and  prisms  of  rocksalt,  of  a  size  sufficient  to  permit 
of  their  being  substituted  for  the  ordinary  glass  train  of  a  Du- 
boscq's  electric  lamp.  A  double  rocksalt  lens  placed  in  the  cam- 
era rendered  the  rays  parallel;  the  parallel  rays  then  passed 
through  a  slit,  and  a  second  rocksalt  lens,  placed  without  the  cam- 
era, produced,  at  an  appropriate  distance,  an  image  of  the  slit. 
Behind  this  lens  was  placed  a  rocksalt  prism,  while  laterally  stood 
a  thermo-electric  pile  intended  to  examine  the  spectrum  produced 
by  the  prism.  Within  the  camera  of  the  electric  lamp  was  placed 
a  burner  with  a  single  aperture,  so  that  the  flame  issuing  from  it 
occupied  the  position  usually  taken  up  by  the  coal-points.  This 
burner  was  connected  with  a  T-piece,  from  which  two  pieces  of 
india-rubber  tubing  were  carried,  the  one  to  a  large  hydrogen- 
holder,  the  other  to  the  gas-pipe  of  the  laboratory.  It  was  thus 
in  my  power  to  have,  at  will,  either  the  gas  flame  or  the  hydrogen 
flame.  When  the  former  was  employed,  I  had  a  visible  spectrum, 
which  enabled  me  to  fix  the  thermo-electric  pile  in  its  proper  posi- 
tion. To  obtain  the  hydrogen  flame,  it  was  only  necessary  to  turn 
on  the  hydrogen  until  it  reached  the  gas  flame  and  was  ignited ; 
then  to  turn  off  the  gas  and  leave  the  hydrogen  flame  behind.  In 
this  way,  indeed,  the  one  flame  could  be  substituted  for  the  other 
without  opening  the  door  of  the  camera,  or  producing  any  change 
in  the  positions  of  the  instruments. 

The  thermo-electric  pile  employed  is  a  beautiful  instrument 
constructed  by  Kuhmkorff.  It  belongs  to  my  friend  Mr.  Gassiot, 
and  consists  of  a  single  row  of  elements  properly  mounted  and 
attached  to  a  double  brass  screen.  It  has  in  front  two  silvered 
edges,  which,  by  means  of  a  screw,  can  be  caused  to  close  upon 
the  pile,  so  as  to  render  its  face  as  narrow  as  desirable,  reducing  it 
to  the  width  of  the  finest  hair,  or,  indeed,  shutting  it  off  altogether. 
By  means  of  a  small  handle  and  long  screw,  the  plate  of  brass  and 
the  pile  attached  to  it  can  be  moved  gently  to  and  fro,  and  thus 
the  vertical  slit  of  the  pile  can  be  caused  to  traverse  the  entire 
spectrum,  or  to  pass  beyond  it  in  both  directions.  The  width  of 
the  spectrum  was  in  each  case  equal  to  the  length  of  the  face  of 
the  pile,  which  was  connected  with  an  extremely  delicate  galvan- 
ometer. 

I  began  with  a  luminous  gas  flame,  the  spectrum  being  cast 
20 


458  APPENDIX   TO   LECTUEE   XII. 

upon  the  brass  screen  (which,  to  render  the  colours  more  visible, 
was  covered  with  tinfoil),  the  pile  was  gradually  moved  in  the  di- 
rection from  blue  to  red,  until  the  deflection  of  the  galvanometer 
became  a  maximum.  To  reach  this  it  was  necessary  to  pass  en- 
tirely through  the  spectrum  and  beyond  the  red ;  the  deflection 
then  observed  was 

30°. 

"When  the  pile  was  moved  in  either  direction  from  this  position,  the 
deflection  diminished. 

The  hydrogen  flame  was  now  substituted  for  the  gas  flame ;  the 
visible  spectrum  disappeared,  and  the  deflection  fell  to 

12°. 

Hence,  as  regards  rays  of  this  peculiar  refrangibility,  the  emission 
from  the  luminous  gas  flame  was  two  and  a  half  times  that  from 
the  hydrogen  flame. 

The  pile  was  now  moved  to  and  fro,  the  movement  in  both  di- 
rections being  accompanied  by  a  diminished  deflection.  Twelve 
degrees,  therefore,  was  the  maximum  deflection  for  the  hydrogen 
flame;  and  the  position  of  the  pile,  determined  previously  by 
means  of  the  luminous  flame,  proves  that  this  deflection  was  pro- 
duced by  extra-red  undulations.  I  moved  the  pile  a  little  for- 
wards, so  as  to  reduce  the  deflection  from  12°  to  4°,  and  then,  in 
order  to  ascertain  the  refrangibility  of  the  rays  which  produced 
this  small  deflection,  I  relighted  the  gas.  The  rectilinear  face  of 
the  pile  was  found  invading  the  red.  When  the  pile  was  caused  to 
pass  successively  through  positions  corresponding  to  the  various 
colours  of  the  spectrum,  and  to  its  extra- violet  rays,  no  measurable 
deflection  was  produced  by  the  hydrogen  flame. 

I  next  placed  the  pile  at  some  distance  from  the  invisible  spec- 
trum of  the  flame  of  hydrogen,  and  felt  for  the  spectrum  by  mov- 
ing the  pile  to  and  fro.  Having  found  it,  I  without  difficulty  as- 
certained the  place  of  maximum  heating.  Changing  nothing  else, 
I  substituted  the  luminous  flame  for  the  non-luminous  one ;  the  po- 
sition of  the  pile,  when  thus  revealed,  was  beyond  the  red. 

It  is  thus  proved  that  the  radiation  from  a  hydrogen  flame  is 
sensibly  extra-red.  The  other  constituents  of  the  radiation  are  so 


LUMINOUS   AND   OBSCUEE   EADIATION.  459 

feeble  as  to  be  thermally  insensible.  Hence,  when  a  body  is  raised 
to  incandescence  by  a  hydrogen  flame,  the  vibrating  periods  of  its 
atoms  must  be  more  rapid  than  those  to  which  the  radiation  of  the 
flame  itself  is  due. 

The  falling  of  the  deflection  from  30°  to  12°,  when  the  hydro- 
gen flame  was  substituted  for  the  gas  flame,  is  doubtless  due  to  the 
absence  of  ah1  solid  matter  in  the  former.  We  may,  however,  in- 
troduce such  matter,  and  thus  make  the  radiation  originating  in 
the  hydrogen  flame  much  greater  than  that  of  the  gas  flame.  A 
spiral  of  platinum  wire  plunged  in  the  former  gave  a  maximum 
deflection  of 

52°, 

at  a  time  when  the  maximum  deflection  of  the  gas  flame  was  only 

33°. 

It  is  mainly  by  convection  that  the  hydrogen  flame  disperses  its 
heat :  though  its  temperature  is  higher,  its  sparsely  scattered  mole- 
cules are  not  able  to  cope,  in  radiant  energy,  with  the  solid  carbon 
of  the  luminous  flame.  The  same  is  true  for  the  flame  of  a  Bun- 
sen's  burner ;  the  moment  the  air  (which  destroys  the  solid  carbon 
particles)  mingles  with  the  gas  flame,  the  radiation  falls  considera- 
bly. Conversely,  a  gush  of  radiant  heat  accompanies  the  shutting 
out  of  the  air  which  deprives  the  gas  flame  of  its  luminosity. 
"When,  therefore,  we  introduce  a  platinum  wire  into  a  hydrogen 
flame,  or  carbon  particles  into  a  Bunsen's  flame,  we  obtain  not  only 
waves  of  a  new  period,  but  also  convert  a  large  portion  of  the  heat 
of  convection  into  the  heat  of  radiation. 

The  action  was  still  very  sensible  when  the  distance  of  the  pile 
from  the  red  end  of  the  spectrum,  on  the  one  side,  was  as  great  as 
that  of  the  violet  rays  on  the  other,  the  heat  spectrum  thus  prov- 
ing itself  to  be  at  least  as  long  as  the  light  spectrum. 

Bunsen  and  Kirchhoff  have  proved  that,  for  incandescent  me- 
tallic vapours,  the  period  of  vibration  is,  within  wide  limits,  inde- 
pendent of  temperature.  My  own  experiments  with  flames  of  hy- 
drogen and  carbonic  oxide  as  sources,  and  with  cold  aqueous 
vapour  and  cold  carbonic  acid  as  absorbing  media,  point  to  the 
same  conclusion.  But  in  solid  metals  augmented  temperature 


460  APPENDIX   TO   LEOTUKE   XII. 

introduces  waves  of  shorter  periods  into  the  radiation.  It  may 
be  asked,  'What  becomes  of  the  long  obscure  periods  when  we 
heighten  the  temperature  ?  Are  they  broken  up  or  changed  into 
shorter  ones,  or  do  they  maintain  themselves  side  by  side  with  the 
new  vibrations? '  The  question  is  worth  an  experimental  answer. 

A  spiral  of  platinum  wire,  suitably  supported,  was  placed 
within  the  camera  of  the  electric  lamp  at  the  place  usually  occu- 
pied by  the  carbon  points.  This  spiral  was  connected  with  a  vol- 
taic battery ;  and  by  varying  the  resistance  to  the  current,  it  was 
possible  to  raise  the  spiral  gradually  from  a  state  of  darkness  to  an 
intense  white  heat.  Eaising  it  to  a  white  heat  in  the  first  instance, 
the  rocksalt  train  was  placed  in  the  path  of  its  rays,  and  a  brilliant 
spectrum  was  obtained.  The  pile  was  then  moved  into  the  posi- 
tion of  maximum  heat  beyond  the  red  of  the  spectrum.  Altering 
nothing  but  the  strength  of  the  current,  the  spiral  was  reduced  to 
darkness,  and  lowered  in  temperature  till  the  deflection  of  the  gal- 
vanometer fell  to  1°.  Our  question  is,  'What  becomes  of  the 
waves  which  produce  this  deflection  when  new  ones  are  introduced 
by  augmenting  the  temperature  of  the  spiral  ? ' 

Causing  the  spiral  to  pass  from  this  state  of  darkness,  through 
various  degrees  of  incandescence,  the  following  deflections  were 
obtained : — 

Appearance  of  spiral  Deflection  by  obscure  rays 

Dark 1° 

Dark 6 

Faint  red  ....  10-4 
Dull  red  .  .  .  .12-5 

Ked 18-0 

Full  red 27'0 

Bright  red  ....  44'4 
Nearly  white  ....  54-3 
Full  white  .  .  .  .  60-0 

The  deflection  of  60°  here  obtained  is  equivalent  to  122  of  the 
fir3t  degrees  of  the  galvanometer.  Hence  the  intensity  of  the  ob- 
scure rays,  in  the  caso  of  the  full  white  heat,  is  122  times  that  of 
the  rays  of  the  same  refrangibility  emitted  by  the  dark  spiral  used 
at  the  commencement.  Or,  as  the  intensity  is  proportional  to  the 
square  of  the  amplitude,  the  height  of  the  ethereal  waves  which 
produced  the  last  deflection  was  eleven  times  that  of  the  waves 


LUMINOUS   AND   OBSCUEE   EADIATION.  461 

which  produced  the  first.  The  wave-length,  of  course,  remained 
the  same  throughout. 

The  experimental  answer,  therefore,  to  the  question  above  pro- 
posed is,  that  the  amplitude  of  the  old  waves  is  augmented  by  the 
same  accession  of  temperature  that  gives  birth  to  the  new  ones. 
The  case  of  the  obscure  rays  is,  in  fact,  that  of  the  luminous  ones 
(of  the  red  of  the  spectrum,  for  example),  which  glow  with  aug- 
mented intensity,  as  the  temperature  of  the  radiant  source  is  height- 
ened. 

In  my  last  memoir  *  I  demonstrated  the  wonderful  transparency 
of  the  element  iodine  to  the  extra-red  undulations.  A  perfectly 
opaque  solution  of  this  substance  was  obtained  by  dissolving  it  in 
bisulphide  of  carbon ;  and  it  was  shown  in  the  memoir  referred  to, 
that  a  quantity  of  iodine,  sufficient  to  quench  the  light  of  our  most 
brilliant  flames,  transmitted  99  per  cent,  of  the  radiation  from  a 
flame  of  hydrogen. 

Fifty  experiments  on  the  radiant  heat  of  a  hydrogen  flame,  re- 
cently executed,  make  the  transmission  of  its  rays,  through  a  quan- 
tity of  iodine  which  is  perfectly  opaque  to  light, 

100  per  cent. 

To  the  radiation  from  a  hydrogen  flame  the  dissolved  iodine  is 
therefore,  according  to  these  experiments,  perfectly  transparent. 

It  is  also  sensibly  transparent  to  the  radiation  from  solid  bodies 
heated  under  incandescence. 

It  is  also  sensibly  transparent  to  the  obscure  rays  emitted  by 
luminous  bodies. 

To  the  mixed  radiation  which  issues  from  solid  bodies  at  a  very 
high  temperature,  the  pure  bisulphide  of  carbon  is  also  eminently 
transparent.  Hence,  as  the  bisulphide  of  carbon  interferes  but 
slightly  with  the  obscure  rays  issuing  from  a  highly  luminous 
source,  and  as  the  dissolved  iodine  seems  not  at  all  to  interfere 
with  them,  we  have  in  a  combination  of  both  substances  a  means 
of  almost  entirely  detaching  the  purely  thermal  rays  from  the 
luminous  ones. 

If  vibrations  of  a  long  period,  established  when  the  radiating 
body  is  at  a  low  temperature,  maintain  themselves,  as  before  indi- 

*  Phil.  Trans,  vol.  cliv.  p.  327.     Phil.  Mag.,  Dec.  1864. 


462  APPENDIX   TO    LECTURE   XII. 

cated,  side  by  side  with  the  new  periods  which  augmented  temper- 
ature introduces,  it  would  follow,  that  a  body  once  pervious  to  the 
radiation  from  any  source,  must  always  remain  pervious  to  it.  We 
cannot  so  alter  the  character  of  the  radiation,  that  a  body  once  in 
any  measure  transparent  to  it,  shall  become  quite  opaque  to  it. 
We  may,  by  augmenting  the  temperature,  diminish  the  percentage 
of  the  total  radiation  transmitted  by  the  body ;  but  inasmuch  as 
the  old  vibrations  have  their  amplitudes  enlarged  by  the  very  ac- 
cession of  temperature  which  produces  the  new  ones,  the  total 
quantity  of  heat  of  any  given  refrangibility,  transmitted  by  the 
body,  must  increase  with  increase  of  temperature. 

This  conclusion  is  thus  experimentally  illustrated.  A  cell  with 
parallel  sides  of  polisted  rocksalt  was  filled  with  the  solution  of 
iodine,  and  placed  in  front  of  the  camera  within  which  was  the 
platinum  spiral  heated  by  a  voltaic  current.  Behind  the  rocksalt 
cell  was  placed  an  ordinary  thermo-electric  pile,  to  receive  such 
rays  as  had  passed  through  the  solution.  The  rocksalt  lens  was  in 
the  camera  in  front,  but  a  small  sheaf  only  of  the  parallel  beam, 
emergent  from  the  lamp,  was  employed.  Commencing  at  a  very 
low  dark  heat,  the  temperature  was  gradually  augmented  to  full 
incandescence  with  the  following  results : — 

Appearance  of  spiral  Deflection 

Dark 1° 

Dark  but  hotter  .  .  .  3 
Dark  but  still  hotter  .  .  5 
Dark  but  still  hotter  .  .  10 
Feeble  red  .  .  .  .19 
Dull  red.  ....  25 

Red 35 

Full  red 45 

Bright  red       .         .         .         .53 
Very  bright  red       .        .         .     63  - 
Nearly  white    ....     69 

White 75 

Intense  white  .        .        .80 

To  the  luminous  rays  from  the  intensely  white  spiral,  the  solu- 
tion was  perfectly  opaque ;  but  though  by  the  introduction  of  such 
rays,  the  transmission,  as  expressed  in  parts  of  the  total  radiation, 
was  diminished,  the  quantity  absolutely  transmitted  was  enor- 


LUMINOUS   AND   OBSCURE   RADIATION.  463 

mously  increased.  The  value  of  the  last  deflection  is  440  times 
that  of  the  first ;  by  raising  therefore  the  platinum  spiral  from  dark- 
ness to  whiteness,  we  augment  the  intensity  of  the  obscure  rays 
which  it  emits  in  the  ratio  of  1 :  440. 

A  rocksalt  cell,  tilled  with  the  transparent  bisulphide  of  carbon, 
was  placed  in  front  of  the  camera  which  contained  the  dazzling 
white-hot  platinum  spiral.  The  transparent  liquid  was  then  drawn 
off  and  its  place  supplied  by  the  solution  of  iodine.  The  deflec- 
tions observed  in  the  respective  cases  are  as  follows : — 

KADIATION  FROM  WHITE-HOT  PLATINUM. 
Through  transparent  CSa  Through  opaque  solution 

73-9°  73-0° 

73-8  72-9 

All  the  luminous  rays  passed  through  the  transparent  bisulphide, 
none  of  them  passed  through  the  solution  of  iodine.  Still  we  see 
what  a  small  difference  is  produced  by  their  withdrawal.  The 
actual  proportion  of  luminous  to  obscure  rays,  as  calculated  from 
the  above  observations,  may  be  thus  expressed : — 

Dividing  the  radiation  from  a  platinum  wire  raised  to  a  daz- 
zling whiteness  l)y  an  electric  current  into  twenty-four  equal  parts, 
one  of  those  parts  is  luminous,  and  twenty -three  olscure. 

A  bright  gas  flame  was  substituted  for  the  platinum  spiral,  the 
top  and  bottom  of  the  flame  being  shut  off,  and  its  most  brilliant 
portion  chosen  as  the  source  of  rays.  The  result  of  forty  experi- 
ments with  this  source  may  be  thus  expressed : — 

Dividing  the  radiation  from  the  most  brilliant  portion  of  a 
flame  of  coal  gas  into  twenty-Jive  equal  parts,  one  of  those  parts  is 
luminous  and  twenty-four  obscure. 

I  next  examined  the  ratio  of  obscure  to  luminous  rays  in  the 
electric  light.  A  battery  of  fifty  cells  was  employed,  and  the  rock- 
salt  lens  was  used  to  render  the  rays  from  the  coal  points  parallel. 
To  prevent  the  deflection  from  reaching  an  inconvenient  magni- 
tude, the  parallel  rays  were  caused  to  pass  through  a  circular  aper- 
ture O'l  of  an  inch  in  diameter,  and  were  sent  alternately  through 
the  transparent  bisulphide  and  the  opaque  solution.  It  is  not  easy 
to  obtain  perfect  steadiness  on  the  part  of  the  electric  light ;  but 
three  experiments  carefully  executed  gave  the  following  deflec- 
tions : — 


464  APPENDIX   TO   LECTURE   XII. 

RADIATION  FROM  ELECTRIC  LIGHT. 

Through  Through 

transparent  CSz  opaque  solution 

Experiment  No.  I.        .        .     72 -0°  70-0° 

Experiment  No.  II.       .         .     76-5      •  75-0 

Experiment  No.  III.     .        .     77'5  76'5 

Calculating  from  these  measurements  the  proportion  of  luminous 
to  obscure  heat,  the  result  may  be  thus  expressed : — 

Dividing  the  radiation  from  the  electric  light  emitted  ly  carbon 
points,  and  excited  ly  a  Grove's  lattery  of  forty  cells,  into  ten 
equal  parts,  one  of  those  parts  is  luminous  and  nine  obscure. 

The  results  may  be  thus  presented  in  a  tabular  form : — 

RADIATION  THROUGH  DISSOLVED  IODINE.* 
Source  Absorption  Transmission 

Dark  spiral         ....       0  100 

Lampblack  at  212°  Fahr.  0  100 

Red-hot  spiral    ....       0  100 

Hydrogen  flame          ...      0  100 

Oil  flame 3  97 

Gas  flame          ....      4  96 

White-hot  spiral         .        .        .4-6  95'4 

Electric  light     ....     10  90 

Future  experiments  may  slightly  alter  these  results,  but  they 
are  extremely  near  the  truth. 

Having  thus,  in  the  solution  of  iodine,  found  a  means  of  almost 
perfectly  detaching  the  obscure  from  the  luminous  heat-rays  of  any 
source,  we  are  able  to  operate  at  will  upon  the  former.  Here  are 
some  illustrations : — The  rocksalt  lens  was  so  placed  in  the  camera 
that  the  coal-points  themselves,  and  their  image  beyond  the  lens, 
were  equally  distant  from  the  latter.  A  battery  of  forty  cells  be- 
ing employed,  the  track  of  the  cone  of  rays  emergent  from  the 
lamp  was  plainly  seen  in  the  air,  and  their  point  of  convergence 
therefore  easily  fixed.  The  cell  containing  the  opaque  solution  was 
now  placed  in  front  of  the  lamp.  The  luminous  cone  was  thereby 
entirely  cut  off,  but  the  intolerable  temperature  of  the  focus,  when 
the  hand  was  placed  there,  showed  that  the  calorific  rajs  were 

*  In  these  experiments,  the  pure  bisulphide  was  compared  with  the 
opaque  solution ;  the  transmission  100  means  that  the  same  quantity  of 
heat  passed  through  both.  The  iodine  was,  in  this  case,  transparent. 


LUMINOUS    AND   OBSCURE   RADIATION.  465 

still  transmitted.  Thin  plates  of  tin  and  zinc  were  placed  succes- 
sively in  the  dark  focus  and  speedily  fused ;  matches  were  ignited, 
gun-cotton  exploded,  and  brown  paper  set  on  fire.  Employing  the 
iodine  solution  and  a  battery  of  sixty  of  Grove's  cells,  all  these  re- 
sults were  readily  obtained  with  the  ordinary  glass  lenses  of  Du- 
boscq's  electric  lamp.  They  cannot,  I  think,  fail  to  give  pleasure  to 
those  who  repeat  the  experiments.  It  is  extremely  interesting  to 
observe  in  the  middle  of  the  air  of  a  perfectly  dark  room  a  piece 
of  black  paper  suddenly  pierced  by  the  invisible  rays,  and  the  burn- 
ing ring  expanding  on  all  sides  from  the  centre  of  ignition. 

On  the  15th  of  this  month  (November,  1864)  I  made  a  few  ex- 
periments on  solar  light.  The  heavens  were  not  free  from  clouds, 
nor  the  London  atmosphere  from  smoke,  and  at  best  I  obtained 
only  a  portion  of  the  action  which  a  clear  day  would  have  given 
me.  I  happened  to  possess  a  hollow  lens,  which  I  filled  with  the 
concentrated  solution  of  iodine.  Placed  in  the  path'of  the  solar 
rays,  a  faint  red  ring  was  imprinted  on  a  sheet  of  white  paper  held 
behind  the  lens,  the  ring  contracting  to  a  faint  red  spot  when  the 
focus  of  the  lens  was  reached.  It  was  immediately  found  that  this 
ring  was  produced  by  the  light  which  had  penetrated  the  thin  rim 
of  the  liquid  lens.  Pasting  a  zone  of  black  paper  round  the  rim, 
the  ring  was  entirely  cut  off  and  no  visible  trace  of  solar  light 
crossed  the  lens.  At  the  focus,  whatever  light  passed  would  be  in- 
tensified nine  hundred  fold ;  still  even  here  no  light  was  visible. 

'  Not  so,  however  with  the  sun's  obscure  rays ;  the  focus  was 
burning  hot.  A  piece  of  black  paper  placed  there  was  instantly 
pierced  and  set  on  fire ;  and  by  shifting  the  paper,  aperture  after 
aperture  was  formed  in  quick  succession.  Gunpowder  was  also 
exploded.  In  fact  we  had  in  the  focus  of  the  sun's  dark  rays  a 
heat  decidedly  more  powerful  than  that  of  the  electric  light,  simi- 
larly condensed,  and  all  the  effects  obtained  with  the  former  could 
be  obtained  in  an  increased  degree  with  the  latter. 

I  introduced  a  plano-convex  lens  of  glass,  larger  than  the 
opaque  lens  just  referred  to,  into  the  path  of  the  sun's  rays.  The 
focus  on  white  paper  was  of  dazzling  brilliancy ;  and  in  this  focus 
the  results  already  described  were  obtained.  I  then  introduced  a 
cell  containing  a  solution  of  alum  in  front  of  the  focus.  The  in- 
tensity of  the  light  at  the  focus  was  not  sensibly  changed ;  still 
these  almost  intolerable  visual  rays,  aided  as  they  were  by  a  con- 
20* 


466  APPENDIX   TO   LECTUPwE   XII. 

siderable  quantity  of  invisible  rays  which  had  also  passed  through 
the  alum,  were  incompetent  to  produce  effects,  which  were  ob- 
tained with  ease  in  the  perfectly  dark  focus  of  the  opaque  lens. 

Thinking  that  this  reduction  of  power  might  be  due  in  part  to 
the  withdrawal  of  heat,  by  reflexion,  from  the  sides  of  the  glass 
cell,  I  put  in  its  place  a  rocksalt  cell  filled  with  the  opaque  solution. 
Behind  this  cell  the  rays  manifested  the  power  which  they  exhib- 
ited in  the  focus  of  the  opaque  lens. 

Melloni's  experiments  led  him  to  conclude  that  rocksalt  trans- 
mits obscure  and  luminous  rays  equally  well,  and  that  a  solution  of 
alum  of  moderate  thickness  entirely  intercepts  the  invisible  rays, 
while  it  allows  all  the  luminous  ones  to  pass.  Hence  the  differ- 
ence between  the  transmissions  of  rocksalt  and  alum  ought  to  give 
the  obscure  radiation.  In  this  way  Melloni  found  that  10  per  cent, 
only  of  the  radiation  from  an  oil  flame  consists  of  luminous  rays. 
The  method  above  employed  proves  that  the  proportion  of  lumi- 
nous heat  to  obscure,  in  the  case  of  an  oil  flame,  is  probably  not 
more  than  one-third  of  what  Melloni  made  it. 

In  fact  this  distinguished  man  clearly  saw  the  possible  inaccu- 
racy of  the  conclusion,  that  none  but  luminous  rays  are  transmit- 
ted by  alum  ;  and  the  following  experiments  justify  the  causes  of 
limitation  which  he  attached  to  his  conclusion : — 

The  solution  of  iodine  was  placed  in  front  of  the  electric  lamp, 
the  luminous  rays  being  thereby  intercepted.  Behind  the  rocksalt 
cell  containing  the  opaque  solution  was  placed  a  glass  cell,  empty 
in  the  first  instance.  The  deflection  produced  by  the  obscure  rays 
which  passed  through  both  produced  a  deflection  of 

80°. 

The  glass  cell  was  now  filled  with  a  concentrated  solution  of  alum ; 
the  deflection  produced  by  the  obscure  rays  passing  through  both 
solutions  was 

50°. 

Calculating  from  the  values  of  these  deflections,  it  was  found  that 
of  the  obscure  heat  emergent  from  the  solution  of  iodine,  20  per 
cent,  was  transmitted  by  the  alum. 

A  point  of  very  considerable  importance  forces  itself  upon  our 
attention  here,  namely,  the  vast  practical  difference  which  may 


LUMINOUS   AND   OBSCURE   RADIATION.  467 

exist  between  the  two  phrases,  '  obscure  rays,'  and  '  rays  from  an 
obscure  source.'  Many  writers  seem  to  regard  these  phrases  as 
equivalent  to  each  other,  and  are  thus  led  into  grave  errors.  A 
stratum  of  alum  solution  ^th  of  an  inch  in  thickness  is,  according 
to  Mclloni,  entirely  opaque  to  the  radiation  from  all  bodies  heated 
under  incandescence.  In  the  foregoing  experiments  the  layer  of 
alum  solution  traversed  by  the  obscure  rays  of  our  luminous  source, 
was  thirty  times  the  thickness  of  the  layer  which  Melloni  found 
sufficient  to  quench  all  rays  emanating  from  obscure  sources. 

There  cannot  be  a  doubt  that  the  invisible  rays  which  have 
shown  themselves  competent  to  traverse  such  a  thickness  of  the 
most  powerful  adiathermic  liquid  yet  discovered  are  also  able  to 
pass  through  the  humours  of  the  eye.  The  very  careful  and  inter- 
esting experiments  of  M.  Janssen,*  prove  that  the  humours  of  the 
eye  absorb  an  amount  of  radiant  heat  exactly  equal  to  that  ab- 
sorbed by  a  layer  of  water  of  the  same  thickness,  and  in  our  solu- 
tion the  power  of  alum  is  added  to  that  of  water.  Direct  experi- 
ments on  the  vitreous  humour  of  an  ox  lead  me  to  conclude,  that 
one-fifth  of  the  obscure  rays  emitted  by  an  intense  electric  light 
reach  the  retina ;  and,  inasmuch  as  in  every  ten  equal  parts  of  the 
radiation  from  an  electric  lamp  nine  consist  of  obscure  rays,  it  fol- 
lows that,  in  the  case  of  the  electric  light,  nearly  two-thirds  of  the 
whole  radiant  energy  which  actually  reaches  the  retina  is  incom- 
petent to  excite  vision.  TFith  a  white-hot  platinum  spiral  as 
source,  the  mean  of  four  good  experiments  gave  a  transmission  of 
11 '7  per  cent,  of  the  obscure  heat  of  the  spiral  through  a  layer  of 
distilled  water  1-2  inch  in  thickness.  A  larger  proportion  no  doubt 
reaches  the  retina.f 

Converging  the  beam  from  the  electric  lamp  by  a  glass  lens,  I 
placed  the  opaque  solution  of  iodine  before  my  open  eye,  and 
brought  the  eye  into  the  focus  of  obscure  rays.  The  he^at  was  im- 
mediately unbearable.  But  it  seemed  to  me  that  the  unpleasant 
effect  was  mainly  due  to  the  action  of  the  obscure  rays  upon  the 
eyelids  and  other  opaque  parts  round  the  eye.  I  therefore  cut,  in 
a  card,  an  aperture  somewhat  larger  than  the  pupil,  and  allowed 
the  concentrated  calorific  beam  to  enter  my  eye  through  this  aper- 

*  Annales  de  CUmie  et  de  Physique,  torn.  Ix.  p.  71. 
f  M.  Franz  has  shown  that  a  portion  of  the  sun's  obscure  rays  reach 
the  retina. 


468  APPENDIX   TO   LECTURE   XII. 

ture.  The  sense  of  heat  entirely  disappeared.  Not  only  were  tlio 
rays  thus  received  upon  the  retina  incompetent  to  excite  vision,  hut 
the  optic  nerve  seemed  unconscious  of  their  existence  even  as  heat. 
What  the  consequence  would  have  been  had  I  permitted  the  lumi- 
nous third  of  the  condensed  beam  to  enter  my  eye,  I  am  not  pre- 
pared to  say,  nor  should  I  like  to  make  the  experiment. 

On  a  tolerably  clear  night  a  candle-flame  can  be  readily  seen  at 
the  distance  of  a  mile.  The  intensity  of  the  electric  light  used  "by 
me  is  650  times  that  of  a  good  composite  candle,  and  as  the  'non- 
luminous  radiation  from  the  coal-points  which  reaches  the  retina 
is  equal  to  twice  the  luminous,  it  follows  that  at  a  common  dis- 
tance of  a  foot,  the  energy  of  the  invisible  rays  of  the  electric 
light  which  reach  the  optic  nerve,  but  are  incompetent  to  provoke 
vision,  is  1,300  times  that  of  the  light  of  a  candle.  But  the  inten- 
sity of  the  candle's  light  at  the  distance  of  a  mile  is  less  than  one 
twenty-millionth  of  its  intensity  at  the  distance  of  a  foot ;  hence 
the  energy  which  renders  the  candle  perfectly  visible  a  mile  ofi°, 
would  have  to  be  multiplied  by  1,300  x  20,000,000,  or  by  twenty-six 
thousand  millions,  to  bring  it  up  to  the  intensity  of  that  invisible 
radiation  which  the  retina  receives  from  the  electric  light  at  a  foot 
distance.  Nothing,  I  think,  could  more  forcibly  illustrate  the  spe- 
cial relationship  which  subsists  between  the  optic  nerve  and  the 
oscillating  periods  of  luminous  bodies.  The  nerve,  like  a  musical 
string,  responds  to  the  periods  with  which  it  is  in  accordance,  while 
it  refuses  to  be  excited  by  others  of  vastly  greater  energy,  which 
are  not  in  unison  with  its  own. 

By  means  of  the  opaque  solution  of  iodine,  I  have  already 
shown  that  the  quantity  of  luminous  heat  emitted  by  a  bright  red 
platinum  spiral  is  immeasurably  small.*  Here  are  some  determina- 
tions since  made,  with  the  same  source  of  heat  and  a  solution  of 
iodine  in  iydide  of  ethyl,  the  strength  and  thickness  of  the  solution 
being  such  as  entirely  to  intercept  the  luminous  rays : — 

RADIATION  FKOM  RED-HOT  PLATINUM  SPIRAL 

Through  transparent  liquid  Through  opaque  solution 

43-7°  43-7° 

43-7  43-7 

These  experiments  were  made  with  exceeding  care,  and  all  the 
*  Phil.  Trans,  vol.  cliv.  p.  327. 


LUMINOUS   AND   OBSCUKE   RADIATION.  469 

conditions  were  favourable  to  the  detection  of  the  slightest  differ- 
ence in  the  amount  of  heat  reaching  the  galvanometer ;  still  the 
quantity  of  heat  transmitted  by  the  opaque  solution  was  found  to 
be  the  same  as  that  transmitted  by  the  transparent  one.  In  other 
words,  the  luminous  radiation  intercepted  by  the  former,  though 
competent  to  excite  vividly  the  sense  of  vision,  was,  when  ex- 
pressed in  terms  of  actual  energy,  absolutely  immeasurable. 

And  here  we  have  the  solution  of  various  difficulties  which 
from  time  to  time  have  perplexed  experimenters.  When  we  see  a 
vivid  light  incompetent  to  affect  our  most  delicate  thermoscopic 
apparatus,  the  idea  naturally  presents  itself  that  light  and  heat 
must  be  totally  different  things.  The  pure  light  emerging  from  a 
combination  of  water  and  green  glass,  even  when  rendered  intense 
by  concentration,  has,  according  to  Melloni,  no  sensible  heating 
power.*  The  light  of  the  moon  is  also  a  case  in  point.  Concen- 
trated by  a  polyzonal  lens  more  than  a  yard  in  diameter  upon  the 
face  of  his  pile,  it  required  all  Melloni's  acuteness  to  nurse  the  cal- 
orific action  up  to  a  measurable  quantity.  Such  experiments,  how- 
ever, demonstrate,  not  that  the  two  agents  are  dissimilar,  but  that 
the  sense  of  vision  can  be  excited  by  an  amount  of  force  almost 
infinitely  small. 

Here  also  we  are  able  to  offer  a  remark  as  to  the  applicability 
of  radiant  heat  to  fog-signalling.  The  proposition,  in  the  abstract, 
is  a  philosophical  one;  for  were  our  fogs  of  a  physical  character 
similar  to  that  of  the  iodine  held  in  solution  by  the  bisulphide  of 
carbon,  or  to  that  of  iodine  or  bromine  vapour,  it  would  be  possi- 
ble to  transmit  through  them  powerful  fluxes  of  radiant  heat,  even 
after  the  entire  stoppage  of  the  light  from  our  signal  lamps.  But 
our  fogs  are  not  of  this  character.  They  are  unfortunately  so  con- 
stituted as  to  act  very  destructively  upon  the  purely  calorific  rays ; 
and  this  fact,  taken  in  conjunction  with  the  marvellous  sensitive- 
ness of  the  eye,  leads  to  the  conclusion  that  long  before  the  light 
of  our  signals  ceases  to  be  visible,  their  radiant  heat  has  lost  the 
power  of  affecting,  in  any  sensible  degree,  the  most  delicate  ther- 
moscopic apparatus  that  we  could  apply  to  their  detection.! 

*  Taylor's  Scientific  Memoirs,  vol.  i.  p.  392. 

f  Since  the  publication  of  this  memoir,  I  have  greatly  intensified  the 
effects  produced  by  invisible  rays. 


LECTURE    XIII. 

[April  10,  1862.] 

DEW, — A  CLEAR  SKY  AND  CALM  BUT  DAMP  ATMOSPHERE  NECESSARY  FOR 
ITS  COPIOUS  FORMATION — DEWED  SUBSTANCES  COLDER  THAN  UNDEWED 
ONES — DEWED  SUBSTANCES  BETTER  RADIATORS  THAN  UNDEWED  ONES — 
DEW  IS  THE  CONDENSATION  OF  THE  ATMOSPHERIC  VAPOUR  ON  SUBSTANCES 
WHICH  HAVE  BEEN  CHILLED  BY  RADIATION — LUNAR  RADIATION — CON- 
STITUTION OF  THE  SUN — THE  BRIGHT  LINES  IN  THE  SPECTRA  OF  THE 
METALS — AN  INCANDESCENT  VAPOUR  ABSORBS  THE  RAYS  WHICH  IT  CAN 
ITSELF  EMIT — KIRCHHOF's  GENERALISATION — FRAUNHOFER's  LINES — SO- 
LAR CHEMISTRY — EMISSION  OF  THE  SUN — IIERSCHEL  AND  POUILLET's  EX- 
PERIMENTS— MAYER'S  METEORIC  THEORY — EFFECT  OF  THE  TIDES  ON  THE 
EARTH'S  ROTATION — ENERGIES  OF  THE  SOLAR  SYSTEM — HELMHOLTZ,  THOM- 
SON, WATERSTON — RELATION  OF  THE  SUN  TO  ANIMAL  AND  VEGETABLE 
LIFE. 

~YT"~T~E  have  learned  that  our  atmosphere  is  always  more 
V  V  or  less  charged  with  aqueous  vapour,  the  condensa- 
tion of  which  forms  our  clouds,  hail,  rain,  and  snow.  I 
have  now  to  direct  your  attention  to  one  particular  case  of 
condensation,  of  great  interest  and  beauty — one,  moreover, 
regarding  which  erroneous  notions  were  for  a  long  time  en- 
tertained. I  refer  to  the  phenomenon  of  Dew.  The 
aqueous  vapour  of  our  atmosphere  is  a  powerful  radiant, 
but  it  is  diffused  through  air  which  usually  exceeds  its  own 
mass  more  than  one  hundred  times.  Not  only,  then,  its 
own  heat,  but  the  heat  of  the  large  quantity  of  air  which 
surrounds  it,  must  be  discharged  by  the  vapour,  before  it 
can  sink  to  its  point  of  condensation.  The  retardation  of 
chilling  due  to  this  cause  enables  good  solid  radiators,  at 
the  earth's  surface,  to  outstrip  the  vapour  in  their  speed  of 


DEW.  471 

refrigeration ;  and  hence  upon  these  bodies  aqueous  vapour 
may  be  condensed  to  liquid,  or  even  congealed  to  hoar- 
frost, while  at  a  few  feet  above  the  surface  it  still  main- 
tains its  gaseous  state.  This  is  actually  the  case  in  the 
beautiful  phenomenon  which  we  have  now  to  examine. 

We  are  indebted  to  a  London  physician  for  a  true 
theory  of  dew.  In  1818  Dr.  Wells  published  his  admirable 
Essay  upon  this  subject.  He  made  his  experiments  in  a 
garden  in  Surrey,  at  a  distance  of  three  miles  from  Black- 
friars  Bridge.  To  collect  the  dew  he  used  little  bundles 
of  wool,  which,  when  dry,  weighed  10  grains  each  ;  and 
having  exposed  them  during  a  clear  night,  the  amount  of 
dew  deposited  on  them  was  determined  by  the  augmenta- 
tion of  their  weight.  He  soon  found  that  whatever  inter- 
fered with  the  view  of  the  sky  from  his  piece  of  wool,  in- 
terfered also  with  the  deposition  of  dew.  He  supported 
a  board  on  four  props ;  on  the  board  he  laid  one  of  his 
wool  parcels,  and  under  it  a  second  similar  one  ;  during  a 
clear  calm  night,  the  former  gained  14  grams  in  weight, 
while  the  latter  gained  only  4.  He  bent  a  sheet  of  paste- 
board like  the  roof  of  a  house,  and  placed  underneath  it  a 
bundle  of  wool  on  the  grass  :  by  a  single  night's  exposure 
the  wool  gained  2  grains  in  weight,  while  a  similar  piece 
of  Avool  exposed  on  the  grass,  but  quite  unshaded  by  the 
roof,  collected  16  grains  of  moisture. 

Is  it  steam  from  the  earth,  or  is  it  fine  rain  from  the 
heavens,  that  produces  this  deposition  of  dew  ?  Both  of 
these  notions  have  been  advocated.  That  it  does  not  arise 
from  the  earth  is,  however,  proved  by  the  observation,  that 
more  moisture  was  collected  on  the  propped  board  than  on 
the  earth's  surface  under  it.  That  it  is  not  a  fine  rain  is 
proved  by  the  fact,  that  the  most  copious  deposition  occur- 
red on  the  clearest  nights. 

Dr.  Wells  next  exposed  thermometers,  as  he  had  done 
his  wool-bundles,  and  found  that  at  those  places  where  the 


472  LECTUEE    XIII. 

dew  fell  most  copiously  the  temperature  sank  lowest.  On 
the  propped  board  already  referred  to,  he  found  the  tem- 
perature 9°  lower  than  under  it ;  beneath  the  pasteboard 
roof  the  thermometer  was  10°  warmer  than  on  the  open 
grass.  He  also  found  that  when  he  laid  his  thermometer 
upon  a  grass  plot,  on  a  clear  night,  it  sank  sometimes  14° 
lower  than  a  similar  thermometer  suspended  in  free  air  at 
a  height  of  4  feet  above  the  grass.  A  bit  of  cotton,  placed 
beside  the  former,  gained  20  grains ;  a  similar  bit,  beside 
the  latter,  only  11  grains  in  weight.  The  lowering  of  the 
temperature  and  the  deposition  of  the  dew  went  hand  in 
hand.  Not  only  did  the  shade  of  artificial  screens  inter- 
fere with  the  lowering  of  the  temperature  and  the  forma- 
tion of  the  dew,  but  a  cloud-screen  acted  in  the  same  man- 
ner. He  once  observed  his  thermometer,  which,  as  it  lay 
upon  the  grass,  showed  a  temperature  12°  lower  than  the 
air  a  few  feet  above  the  grass,  rise,  on  the  passage  of  some 
clouds,  until  it  was  only  2°  colder  than  the  air.  In  fact,  as 
the  clouds  crossed  his  zenith,  or  disappeared  from  it,  the 
temperature  of  his  thermometer  rose  and  fell. 

A  series  of  such  experiments,  conceived  and  executed 
with  singular  clearness  and  skill,  enabled  Dr.  Wells  to  pro- 
pound a  Theory  of  Dew,  which  has  stood  the  test  of  all 
subsequent  criticism,  and  is  now  universally  accepted. 

It  is  an  effect  of  chilling  by  radiation.  '  The  upper 
parts  of  the  grass  radiate  their  heat  into  regions  of  empty 
space,  which,  consequently,  send  no  heat  back  in  return  ;  its 
lower  parts,  from  the  smallness  of  their  conducting  power, 
transmit  little  of  the  earth's  heat  to  the  upper  parts,  which, 
at  the  same  time,  receiving  only  a  small  quantity  from  the 
atmosphere,  and  none  from  any  other  lateral  body,  must 
remain  colder  than  the  air,  and  condense  into  dew  its  watery 
vapour,  if  this  be  sufficiently  abundant  in  respect  to  the 
decreased  temperature  of  the  grass.'  Why  the  vapour 
itself,  being  a  powerful  radiant,  is  not  as  quickly  chilled  as 


THEORY   OF   WELLS. 

the  grass,  I  have  already  explained,  on  the  ground  that  the 
vapour  has  not  only  its  own  heat  to  discharge,  "but  also  that 
of  the  large  mass  of  air  by  which  it  is  surrounded. 

Dew  being  the  result  of  the  condensation  of  atmo- 
spheric vapour,  on  substances  which  have  been  sufficiently 
cooled  by  radiation,  and  as  bodies  differ  widely  in  their 
radiative  powers,  we  may  expect  corresponding  differences 
in  the  deposition  of  dew.  This  Wells  proved  to  be  the 
case.  He  often  saw  dew  copiously  deposited  on  grass  and 
painted  wood,  when  none  could  be  observed  on  gravel 
walks  adjacent.  He  found  plates  of  metal,  which  he  had 
exposed,  quite  dry,  while  adjacent  bodies  were  covered 
with  dew :  in  all  such  cases  the  temperature  of  the  metal 
icas  found  to  be  higher  than  that  of  the  dewed  substances. 
This  is  quite  in  accordance  with  our  knowledge  that  metals 
are  the  worst  radiators.  On  one  occasion  he  placed  a  plate 
of  metal  upon  grass,  and  upon  the  plate  he  laid  a  glass 
thermometer ;  the  thermometer,  after  some  time,  exhibited 
dew,  while  the  plate  remained  dry.  This  led  him  to  sup- 
pose that  the  instrument,  though  lying  on  the  plate,  did  not 
share  its  temperature.  He  placed  a  second  thermometer, 
with  a  gilt  bulb,  beside  the  first ;  the  naked  glass  ther- 
mometer— a  good  radiator — remained  9°  colder  than  its 
companion.  To  determine  the  true  temperature  of  a  body 
is,  I  may  remark,  a  difficult  task :  a  glass  thermometer,  sus- 
pended in  the  air,  will  not  give  the  temperature  of  the  air ; 
its  own  power  as  a  radiant  or  an  absorbent  comes  into 
play.  On  a  clear  day,  when  the  sun  shines,  the  thermom- 
eter will  be  warmer  than  the  air ;  on  a  clear  night,  on  the 
contrary,  the  thermometer  will  be  colder  than  the  air. 
We  have  seen  that  the  passage  of  a  cloud  can  raise  the 
temperature  of  a  thermometer  10  degrees  in  a  few  minutes. 
This  augmentation,  it  is  manifest,  does  not  indicate  a  cor- 
responding augmentation  of  the  temperature  of  the  air,  but 
18* 


4:74  LECTUKE   XIII. 

merely  the  interception  and  reflection,  by  the  cloud,  of  the 
rays  of  heat  emitted  by  the  thermometer 

Dr.  Wells  applied  his  principles  to  the  explanation  of 
many  curious  effects,  and  to  the  correction  of  many  popular 
errors.  Moon  blindness  he  refers  to  the  chill  produced  by 
radiation  into  clear  space,  the  shining  of  the  moon  being 
merely  an  accompaniment  to  the  clearness  of  the  atmos- 
phere. The  putrefying  influence  ascribed  to  the  moon- 
beams is  really  due  to  the  deposition  of  moisture,  as  a  kind 
of  dew,  on  the  exposed  animal  substances.  The  nipping 
of  tender  plants  by  frost,  even  when  the  air  of  the  garden 
is  some  degrees  above  the  freezing  temperature,  is  also  to 
be  referred  to  chilling  by  radiation.  A  cobweb  screen 
would  be  sufficient  to  preserve  them  from  injury.* 

Wells  was  the  first  to  explain  the  formation,  artificially, 
of  ice  in  Bengal,  where  the  substance  is  never  formed  nat- 
urally. Shallow  pits  are  dug,  which  are  partially  filled 
with  straw,  and  on  the  straw  flat  pans,  containing  water 
which  had  been  boiled,  is  exposed  to  the  clear  firmament. 
The  water  is  a  poweful  radiant,  and  sends  off  its  heat 
copiously  into  space.  The  heat  thus  lost  cannot  be  supplied 
from  the  earth — this  source  being  cut  off  by  the  non-con- 
ducting straw.  Before  sunrise  a  cake  of  ice  is  formed  in 
each  vessel.  This  is  the  explanation  of  Wells,  and  it  is,  no 
doubt,  the  true  one.  I  think,  however,  it  needs  supple- 
menting. It  appears,  from  the  description,  that  the  con- 

*  With  reference  to  this  point  we  have  the  following  beautiful  passage 
in  the  Essay  of  Wells : — '  I  had  often,  in  the  pride  of  half  knowledge,  smiled 
at  the  means  frequently  employed  by  gardeners  to  protect  tender  plants 
from  cold,  as  it  appeared  to  me  impossible  that  a  thin  mat,  or  any  such 
flimsy  substance  could  prevent  them  from  attaining  the  temperature  of  the 
atmosphere,  by  which  alone  I  thought  them  liable  to  be  injured.  But 
when  I  had  learned  that  bodies  on  the  surface  of  the  earth  become,  during 
a  still  and  serene  night,  colder  than  the  atmosphere,  by  radiating  their  heat 
to  the  heavens,  I  perceived  immediately  a  just  reason  for  the  practice  which 
I  had  before  deemed  useless.' 


NOCTURNAL   RADIATION. 

dition  most  suitable  for  the  formation  of  ice,  is  not  only  a 
clear  air,  but  a  dry  air.  The  nights,  says  Sir  Robert 
Barker,  most  favourable  for  the  production  of  ice,  are  those 
which  are  clearest  and  most  serene,  and  in  which  very  little 
dew  appears  after  midnight.  I  have  italicised  a  very  sig- 
nificant phrase.  To  produce  the  ice  in  abundance,  the  at- 
mosphere must  not  only  be  clear,  but  it  must  be  compara- 
tively free  from  aqueous  vapour.  When  the  straw  in 
which  the  pans  were  laid  became  wet,  it  was  always  changed 
for  dry  straw,  and  the  reason  Wells  assigned  for  this  was, 
that  the  straw,  by  being  wetted,  was  rendered  more  com- 
pact, and  efficient  as  a  conductor.  This  may  have  been  the 
case,  but  it  is  also  certain  that  the  vapour  rising  from  the 
wret  straw,  and  overspreading  the  pans  like  a  screen,  would 
check  the  chill,  and  retard  the  congelation. 

With  broken  health  Wells  pursued  and  completed  this 
beautiful  investigation ;  and,  on  the  brink  of  the  grave,  he 
composed  his  Essay.  It  is  a  model  of  wise  enquiry  and  of 
lucid  exposition.  He  made  no  haste,  but  he  took  no  rest 
till  he  had  mastered  his  subject,  looking  steadfastly  into  it 
until  it  became  transparent  to  his  gaze.  Thus  he  solved 
his  problem,  and  stated  its  solution  in  a  fashion  which  ren- 
ders his  work  imperishable.* 

Since  his  time  various  experimenters  have  occupied 
themselves  with  the  question  of  nocturnal  radiation ;  but, 
though  valuable  facts  have  been  accumulated,  if  we  ex- 
cept a  supplement  contributed  by  Melloni,  nothing  of  im- 
portance has  been  added  to  the  theory  of  Wells.  Mr. 
Glaisher,  M.  Martins,  and  others,  have  occupied  themselves 
with  the  subject.  The  following  table  contains  some  re- 
sults obtained  by  Mr.  Glaisher,  by  exposing  thermometers 
at  different  heights  above  the  surface  of  a  grass  field.  The 

*  The  tract  of  Wells  is  preceded  by  a  personal  memoir  written  by  him- 
self. It  has  the  solidity  of  an  essay  of  Montaigne. 


476  LECTUKE    XIII. 

chilling  observed,  when  the  thermometer  was  exposed  on 
long  grass,  is  represented  by  the  number  1,000  ;  while  the 
succeeding  numbers  represent  the  relative  chilling  of  the 
thermometers  placed  in  the  positions  indicated : 

Radiation. 

Long  grass  ....  1000 
One  inch  above  the  points  of  the  grass  .  671 
Two  inches  ,,  ,,  .  570 

Three  inches  „  „  .477 

Six  inches  „  „  .     282 

One  foot  „  n    "•     129 

Two  feet  „  „  .       86 

Four  feet  „  „  .       69 

Six  feet  ,,  ,,  .       52 

It  may  be  asked  why  the  thermometer,  which  is  a  good 
radiator,  is  not,  when  suspended  in  free  air,  just  as  much 
chilled  as  at  the  earth's  surface.  Wells  has  answered  the 
question.  It  is  because  the  thermometer,  when  chilled, 
cools  the  air  in  immediate  contact  with  it ;  this  air  .con- 
tracts, becomes  heavy,  and  trickles  downwards,  thus  allow- 
ing its  place  to  be  taken  by  warmer  air.  In  this  way  the 
free  thermometer  is  prevented  from  falling  very  IOAV  be- 
neath the  temperature  of  the  air.  Hence,  also,  the  neces- 
sity of  a  still  night  for  the  copious  formation  of  dew ;  for, 
when  the  wind  blows,  fresh  air  continually  circulates  amid 
the  blades  of  grass,  and  prevents  any  considerable  chilling 
by  radiation. 

When  a  radiator  is  exposed  to  a  clear  sky  it  tends  to 
keep  a  certain  thermometric  distance,  if  I  may  use  the 
term,  between  its  temperature  and  that  of  the  surrounding 
air.  This  distance  will  depend  upon  the  energy  of  the 
body  as  a  radiator,  but  it  is  to  a  great  extent  independent 
of  the  temperature  of  the  air.  Thus  M.  Pouillet  has  proved 
that  in  the  month  of  April,  when  the  temperature  of  the  air 
was  3°*6  C.,  swansdown  fell  by  radiation  to  —  3°-5:  the 
whole  chilling,  therefore,  was  7°'l.  In  the  month  of  June, 


MELLONl's   SUPPLEMENT  TO   THE  THEORY   OF   DEW.     477 

when  the  temperature  of  the  air  was,  7°'75  C.,  the  tempera- 
ture of  the  radiating  swansdown  was  10°'54 :  the  chilling  of 
the  swansdown  by  radiation  is  here  7°'21  ;  almost  precisely 
the  same  as  that  which  occurred  in  April.  Thus,  while  the 
general  temperature  varies  within  wide  limits,  the  differ- 
ence  of  temperature  between  the  radiating  body  and  the 
surrounding  air,  remains  sensibly  constant. 

These  facts  enabled  Melloni  to  make  an  important  addi- 
tion to  the  theory  of  dew.  He  found  that  a  glass  thermom- 
eter, placed  on  the  ground,  is  never  chilled  more  than  2° 
C.  below  an  adjacent  thermometer,  with  silvered  bulb,  which 
hardly  radiates  at  all.  These  2°  C.,  or  thereabouts,  mark 
the  thermometric  distance  above  referred  to,  which  the 
glass  tends  to  preserve  betweeen  it  and  the  surrounding 
air.  But  Six,  Wilson,  Wells,  Parry,  Scoresby,  Glaisher, 
and  others,  have  found  differences  of  more  than  10°  C., 
between  a  thermometer  on  grass,  and  a  second  thermom- 
eter hung  a  few  feet  above  the  grass.  How  is  this  to  be 
accounted  for  ?  Very  simply,  according  to  Melloni,  thus  : 
— The  grass  blades  first  chill  themselves  by  radiation,  2°  C. 
below  the  surrounding  air :  the  air  is  then  chilled  by  con- 
tact with  the  grass,  and  forms  around  it  a  cold  aerial  bath. 
But  the  tendency  of  the  grass  is  to  keep  the  above  constant 
difference  between  its  own  temperature  and  that  of  the 
surrounding  medium.  It  therefore  sinks  lower.  The  air 
sinks  in  its  turn,  being  still  further  chilled  by  contact  with 
the  grass ;  the  grass,  however,  again  seeks  to  re-establish 
the  former  difference ;  it  is  again  followed  by  the  air,  and 
thus,  by  a  series  of  actions  and  reactions,  the  entire  stratum 
of  air  in  contact  with  the  grass  becomes  lowered  far  below 
the  temperature  which  corresponds  to  the  actual  radiative 
energy  of  the  grass. 

So  much  for  terrestrial  radiation ;  that  of  the  moon 
will  not  occupy  us  so  long.  Many  futile  attempts  have 
been  made  to  detect  the  warmth  of  the  moon's  beams.  No 


4:78  LECTURE    XIII. 

doubt  is  entertained  that  every  luminous  ray  is  also  a  heat 
ray  ;  but  the  light-giving  power  is  not  even  an  approximate 
measure  of  the  calorific  energy  of  a  beam.  With  a  large 
polyzonal  lens,  Melloni  converged  an  image  of  the  moon 
upon  his  pile ;  but  he  found  the  cold  of  his  lens  far  more 
than  sufficient  to  mask  the  heat,  if  such  there  were,  of  the 
moon.  He  screened  off  his  lens  from  the  heavens,  placed 
his  pile  in  the  focus  of  the  lens,  waited  until  the  needle 
came  to  zero,  and  then  suddenly  removing  his  screen  al- 
lowed the  concentrated  light  to  strike  his  pile.  The  slight 
air-drafts  in  the  place  of  experiment  were  sufficient  to  dis- 
guise the  effect.  He  then  stopped  the  tube  in  front  of  his 
pile  with  glass  screens,  through  which  the  light  went 
freely  to  the  blackened  face  of  the  pile,  where  it  was  con- 
verted into  heat.  This  heat  could  not  get  back  through  the 
glass  screen,  and  thus  Melloni,  following  the  example  of 
Saussure,  accumulated  his  effects,  and  obtained  a  deflection 
of  3°  or  4°.  The  deflection  indicated  warmth,  and  this  is 
the  only  experiment  which  gives  us  any  positive  evidence 
as  to  the  calorific  action  of  the  moon's  rays.  Incomparably 
less  powerful  than  the  solar  rays  in  the  first  instance,  their 
action  is  first  enfeebled  by  distance,  and,  secondly,  by  the 
fact  that  the  obscure  heat  of  the  moon  is  almost  wholly  ab- 
sorbed by  our  atmospheric  vapour.  Even  such  obscure 
rays  as  might  happen  to  reach  the  earth  would  be  utterly 
cut  off  by  such  a  lens  as  Melloni  made  use  of.  It  might  be 
worth  while  to  make  the  experiment  with  a  metallic  reflec- 
tor, instead  of  with  a  lens.  I  have  myself  tried  a  conical 
reflector  of  very  large  dimensions,  but  have  hitherto  been 
defeated  by  the  unsteadiness  of  the  London  air. 

We  have  now  to  turn  our  thoughts  to  the  source  from 
which  all  terrestrial  and  lunar  heat  is  derived.  This  source 
is  the  sun  ;  for  if  the  earth  has  ever  been  a  molten  sphere, 
which  is  now  cooling,  the  quantity  of  heat  which  reaches 
its  surface  from  within,  has  long  ceased  to  be  sensible. 


SPECTKUM  ANALYSIS  BY  THE  ELECTKIC  LIGHT.      479 

First,  then,  let  us  enquire  what  is  the  constitution  of  this 
wondrous  body,  to  which  we  owe  both  light  and  life. 

Let  us  approach  the  subject  gradually  and  prepare  our 
minds,  by  previous  discipline,  for  the  treatment  of  this 
noble  problem.  You  already  know  how  the  spectrum  of 
the  electric  light  is  formed.  Here  you  have  one  upon  the 
screen,  two  feet  wide  and  eight  long,  with  all  its  magnifi- 
cent gradations  of  colour,  one  fading  into  the  other,  without 
solution  of  continuity.  The  light  from  which  this  spec- 
trum is  derived,  is  emitted  from  the  incandescent  carbon 
points  within  our  electric  lamp.  All  other  solids  give  a 
similar  spectrum.  When  I  raise  this  platinum  wire  to 
whiteness  by  an  electric  current,  and  examine  its  light  by  a 
prism,  I  find  the  same  gradation  of  colours,  and  no  gap 
whatever  between  one  colour  and  the  other.  But  by  in- 
tense heat, — by  the  heat  of  the  electric  lamp,  for  example, 
— I  can  volatilise  that  platinum,  and  throw  upon  the  screen, 
not  the  spectrum  of  the  incandescent  solid,  but  of  its  in- 
candescent vapour.  The  spectrum  is  now  changed  ;  instead 
of  being  a  continuous  gradation  of  colours,  it  consists  of  a 
series  of  brilliant  lines,  separated  from  each  other  by  spaces 
of  darkness. 

I  have  arranged  my  pieces  of  carbon  thus  : — the  lower 
one  is  now  a  cylinder,  about  half  an  inch  in  diameter,  in 
the  top  of  which  I  have  scooped  a  small  hollow ;  in  this 
hollow  I  place  the  metal  which  I  wish  to  examine — say  this 
piece  of  zinc, — and  bring  down  upon  it  the  upper  point. 
The  current  passes ;  I  draw  the  points  apart,  and  you  see 
the  magnificent  arc  that  now  unites  them ;  here  is  its 
magnified  image  upon  the  screen,  a  fine  stream  of  purple 
light  18  inches  long.  That  coloured  space  contains  the 
particles  of  the  zinc  discharged  across  from  carbon  to  car- 
bon ;  these  particles  are  now  oscillating  in  certain  definite 
periods,  and  the  colour  which  we  perceive  is  the  mixture 
of  impressions  due  to  these  oscillations.  Let  us  separate, 


4:80  LECTURE  xm. 

by  a  prism,  the  coloured  stream  into  its  components  ;  here 
they  are,  splendid  bands  of  red  and  blue.  Pray  remember 
the  character  and  position  of  these  bands,  as  I  shall  have 
to  refer  to  them  again  immediately. 

I  interrupt  the  current ;  eject  the  zinc,  and  put  in  its 
place  a  bit  of  copper.  Here  you  see  a  stream  of  green 
light  between  the  carbons,  which  we  will  analyse  as  we 
did  the  light  of  the  zinc.  You  can  see  that  the  spectrum 
of  the  copper  is  different  from  that  of  the  zinc :  here  you 
have  bands  of  brilliant  green,  which  are  absent  from  the 
zinc.  "We  may  therefore  infer,  with  certainty,  that  the  atoms 
of  copper,  in  the  voltaic  arc,  swing  in  periods  different  from 
those  of  zinc.  Let  us  now  see  whether  these  different 
periods  create  any  confusion,  when  we  operate  upon  a  sub- 
stance composed  of  zinc  and  copper, — the  familiar  sub- 
stance brass.  Its  spectrum  is  now  before  you,  and  if  you 
have  retained  the  impression  made  by  our  two  last  experi- 
ments, you  will  recognise  here  a  spectrum  formed  by  the 
superposition  of  the  two  separate  spectra  of  zinc  and  cop- 
per. The  alloy  emits,  without  confusion,  the  rays  peculiar 
to  the  metals  of  which  it  is  composed. 

Every  metal  emits  its  own  system  of  bands,  which  are 
as  characteristic  of  it  as  those  physical  and  chemical  quali- 
ties which  give  it  its  individuality.  By  a  method  of  ex- 
periment sufficiently  refined  we  can  measure,  accurately,  the 
position  of  the  bright  lines  of  every  known  metal.  Ac- 
quainted with  such  lines  we  should,  by  the  mere  inspection 
of  the  spectrum  of  any  single  metal,  be  able  at  once  to 
declare  its  name.  And  not  only  so,  but  in  the  case  of  a 
mixed  spectrum,  we  should  be  able  to  declare  the  constitu- 
ents of  the  mixture  from  which  it  emanated. 

This  is  true,  not  only  of  the  metals  themselves,  but  also 
of  their  compounds,  if  they  be  volatile.  I  place  a  bit  of 
sodium  on  my  lower  cylinder  and  cause  the  voltaic  dis- 
charge to  pass  from  it  to  the  upper  coal-point ;  here  is  the 


METALLIC   VAPOUES.  481 

spectrum  of  the  sodium :  a  single  band  of  brilliant  yel- 
low. If  I  operated  with  sufficient  delicacy  I  should  divide 
that  band  into  two,  with  a  narrow  dark  interval  between 
them.  I  eject  the  sodium  from  the  lamp  and  put  in  its 
place  a  little  common  salt,  or  chloride  of  sodium.  At  this 
high  temperature  the  salt  is  volatile,  and  you  see  the  exact 
yellow  band  produced  by  the  salt  that  was  given  by  the 
metal.  Thus,  also,  by  means  of  the  chloride  of  strontium 
I  produce  the  bands  of  the  metal  strontium ;  by  the  chlo- 
rides of  calcium,  magnesium,  and  lithium,  I  produce  the 
spectra  of  these  respective  metals. 

Here,  finally,  I  have  a  carbon  cylinder  perforated  with 
holes,  into  which  I  have  stuffed  a  mixture  of  all  the  com- 
pounds just  mentioned  ;  and  there  is  the  spectrum  of  the 
mixture  upon  the  screen.  Surely  nothing  more  magnifi- 
cent can  be  imagined.  Each  substance  gives  out  its  own 
peculiar  rays,  and  thus  they  cut  transversely,  the  whole 
eight  feet  of  the  spectrum  into  splendid  parallel  bars  of 
coloured  light.  Having  previously  made  yourselves  ac- 
quainted with  the  lines  emitted  by  all  the  metals,  you 
would  be  able  to  unravel  this  spectrum,  and  to  tell  me  what 
substances  I  have  employed  in  its  production. 

I  make  use  of  the  voltaic  arc  simply  because  its  light 
is  so  intense  as  to  be  visible  to  a  large  audience  like  the 
present,  but  I  might  make  the  same  experiments  with  a 
common  blow-pipe  flame,  which  is  nearly  deprived  of  light 
by  the  admixture  of  air  or  oxygen.  The  introduction  of 
sodium,  or  chloride  of  sodium,  turns  the  flame  yellow; 
strontium  turns  it  red  ;  copper  green,  &c.  The  flames  thus 
coloured,  when  examined  by  a  prism,  show  the  exact  bands 
which  I  have  displayed  before  you  on  the  screen. 

"We  have  already  learned  that  gases  and  vapours  absorb 
the  rays  of  heat,  the  heat  that  we  employed  being  obscure. 
I  have  no  doubt  that  if  those  rays  could  make  an  impres- 
sion upon  the  eye — if  I  could  spread  them  out  before  you 
21 


4:82  LECTUKE    XIII. 

like  the  colours  of  the  spectrum — you  would  find  certain 
classes  of  rays  selected,  in  each  case,  for  destruction,  the 
others  being  allowed  free  passage  through  the  vapours. 
A  famous  experiment  of  Sir  David  Brewster's,  which  I 
will  throw  into  a  form  suited  to  the  lecture  room,  will 
enable  me  to  illustrate  this  power  of  selection  in  the  case 
of  light.  Into  this  cylinder,  the  ends  of  which  are  stopped 
by  plates  of  glass,  I  introduce  a  quantity  of  nitrous  acid 
gas,  the  presence  of  which  is  now  indicated  by  its  rich 
brown  colour.  I  project  a  spectrum  on  the  screen,  eight 
feet  long  and  nearly  two  in  width,  and  I  place  this  cylin- 
der, containing  the  brown  gas,  in  the  path  of  the  beam  as 
it  issues  from  the  lamp.  You  see  the  effect ;  the  contin- 
uous spectrum  is  now  furrowed  by  numerous  dark  bands, 
the  rays  answering  to  which  are  struck  down  by  the  nitric 
gas,  while  it  permits  the  intervening  bands  of  light  to  pass 
without  hindrance. 

We  must  now  take  a  step  in  advance  of  the  principle  of 
reciprocity,  which  I  have  already  enunciated.  Hitherto 
we  have  found  in  gases,  liquids,  and  solids,  that  the  good 
absorber  is  the  good  radiator ;  we  must  now  go  further  and 
state,  that  a  gas  or  vapour,  absorbs  those  precise  rays  which 
it  can  itself  emit  /  the  atoms  which  swing  at  a  certain  rate 
intercept  the  waves  excited  by  atoms  swinging  at  the  same 
rate.  The  atoms  which  vibrate  red  light  will  stop  red 
light;  the  atoms  that  oscillate  yellow  will  stop  yellow; 
those  that  oscillate  green  will  stop  green,  and  so  of  the 
rest.  Absorption,  you  know,  is  a  transference  of  motion 
from  the  ether  to  the  particles  immersed  in  it,  and  the  ab- 
sorption of  any  atom  is  exerted  chiefly  upon  those  waves 
which  arrive  in  periods  that  correspond  with  the  atom's 
own  rate  of  oscillation. 

Let  us  endeavour  to  prove  this  experimentally.  We 
already  know  that  a  sodium  flame,  when  analysed,  gives  us 
a  brilliant  double  band  of  yellow.  Here  is  a  flat  vessel 


ABSORPTION  BY   SODIUM  VAPOUR.  483 

containing  a  mixture  of  alcohol  and  water ;  I  warm  the 
mixture  and  ignite  it :  it  gives  a  flame  which  is  so  feebly 
luminous  as  to  be  scarcely  visible.  I  now  mix  salt  with 
the  liquid,  and  again  ignite  it ;  the  flame,  which  a  moment 
ago  was  scarcely  to  be  seen,  is  now  a  brilliant  yellow.  I 
project  a  continuous  spectrum  upon  the  screen,  and  in  the 
track  of  the  beam,  as  it  issues  from  the  electric  lamp,  I 
place  the  yellow  sodium  flame.  Observe  the  spectrum  nar- 
rowly :  you  see  a  flickering  gray  band  in  the  yellow  of  the 
spectrum  ;  -sometimes  it  is  shaded  deeply  enough  to  show 
you  all  that  the  flame  has,  at  least  in  part,  intercepted  the 
yellow  band  of  the  spectrum  :  it  has  partially  absorbed  the 
precise  light  which  it  can  itself  emit. 

But  I  wish  to  make  the  effect  plainer,  and  therefore 
abandon  the  alcohol  light,  and  proceed  thus :  here  is  a 
Bunsen's  burner,  the  flame  of  which  is  intensely  hot, 
though  it  hardly  emits  any  light.  I  place  the  burner  in 
front  of  the  lamp,  so  that  the  beam,  whose  decomposition 
is  to  form  our  spectrum,  shall  pass  through  the  flame.  I 
have  here  a  little  net  of  platinum  wire,  in  which  I  place  a 
bit  of  the  metal  sodium,  about  the  size  of  a  pea.  I  also  set 
up  a  pasteboard  shade,  which  shall  cut  off  the  light  emitted 
by  the  sodium,  from  the  screen  on  which  the  spectrum 
falls.  And  now  I  am  ready  to  make  the  experiment. 
Here,  then,  in  the  first  place,  is  the  spectrum.  I  now  in- 
troduce the  platinum  net  in  front  of  the  lamp  ;  the  sodium 
instantly  colours  the  flame  intensely  yellow,  and  you  see  a 
shadow  coming  over  the  yellow  of  the  spectrum.  But  the 
effect  is  not  yet  at  its  maximum.  The  sodium  now  sud- 
denly bursts  into  intensified  combustion,  and  there  you  see 
the  yellow  dug  utterly  out  of  the  spectrum,  and  a  bar  of 
intense  darkness  in  its  place.  This  violent  combustion  will 
endure  for  a  little  time.  I  withdraw  the  flame,  the  yellow 
reappears  upon  the  screen ;  I  reintroduce  it,  the  yellow 
band  is  cut  out.  This  I  can  do  ten  times  in  succession, 


LECTUKE    XTTT. 

and  in  the  whole  range  of  optics  I  do  not  think  there  is  a 
more  striking  experiment.  Here,  then,  we  have  conclu- 
sively proved,  that  the  light  which  the  sodium  flame  ab- 
sorbs is  the  precise  light  which  it  can  emit. 

Let  me  be  still  more  precise  in  my  experiment.  The 
yellow  of  the  spectrum  spreads  over  a  widish  interval ;  and 
I  wish  now  to  show  you  that  it  is  the  particular  portion 
of  the  yellow  which  the  sodium  emits,  that  is  absorbed  by 
its  flame.  I  place  a  little  salt  solution  on  the  ends  of  my 
coal  points  ;  you  now  see  the  continuous  spectrum  with  the 
yellow  band  of  the  sodium  brighter  than  the  rest  of  the 
yellow.  It  is  thus  clearly  defined  before  your  eyes.  I 
again  place  the  sodium  flame  in  front,  and  that  particular 
band  which  now  stands  out  from  the  spectrum  is  cut  away 
— a  space  of  intense  gloom  occupying  its  place. 

You  have  already  seen  a  spectrum,  derived  from  a  mix- 
ture of  various  substances,  and  which  was  composed  of  a 
succession  of  sharply  defined  and  brilliant  bars,  separated 
from  each  other  by  intervals  of  darkness.  Could  I  take 
the  mixture  which  produced  that  striped  spectrum,  and 
raise  it,  by  means  of  Bunsen's  burner,  to  a  temperature 
sufficiently  intense  to  render  its  vapours  incandescent ;  on 
placing  its  flame  in  the  path  of  a  beam  producing  a  contin- 
uous spectrum,  I  should  cut  out  of  the  latter  the  precise 
rays  emitted  by  the  components  of  my  mixture.  I  should 
thus,  instead  of  furrowing  my  spectrum  by  a  single  dark 
band,  as  in  the  case  of  sodium,  furrow  it  by  a  series  of 
dark  bands,  equal  in  number  to  the  bright  bands  produced, 
when  the  mixture  itself  was  the  source  of  light. 

I  think  we  now  possess  knowledge  sufficient  to  raise  us 
to  the  level  of  one  of  the  most  remarkable  generalisations 
of  our  age.  When  the  light  of  the  sun  is  properly  decom- 
posed, the  spectrum  is  seen  furrowed  by  innumerable  dark 
lines.  A  few  of  these  were  observed,  for  the  first  time, 
by  Dr.  Wollaston ;  but  they  were  investigated  with  pro- 


485 

found  skill  by  Fraunhofer,  and  called,  after  him,  Fraun- 
hofer's  lines.  It  has  long  been  supposed  that  these  dark 
spaces  were  caused  by  the  absorption  of  the  rays  which 
correspond  to  them,  in  the  atmosphere  of  the  sun  ;  but  no- 
body knew  how.  Having  once  proved  that  an  incandescent 
vapour  absorbs  the  precise  rays  which  it  can  itself  emit, 
and  knowing  that  the  body  of  the  sun  is  surrounded  by  an 
incandescent  photosphere,  the  supposition  at  once  flashes 
on  the  mind,  that  this  photosphere  may  cut  off  those  rays 
of  the  central  incandescent  orb,  which  the  photosphere 
itself  can  emit.  We  are  thus  led  to  a  theory  of  the  con- 
stitution of  the  sun,  which  renders  a  complete  account  of 
the  lines  of  Fraunhofer. 

The  sun  consists  of  a  central  orb,  liquid  or  solid,  of  ex- 
ceeding brightness,  which,  of  itself,  would  give  a  contin- 
uous spectrum,  or  in  other  words,  winch  emits  all  kinds  of 
rays.  These,  however,  have  to  pass  through  the  photo- 
sphere, which  wraps  the  sun  like  a  flame,  and  this  vaporous 
envelope  cuts  off  those  particular  rays  of  the  nucleus  which 
it  can  itself  emit — the  lines  of  Fraunhofer  marking  the 
position  of  these  failing  rays.  Could  we  abolish  the  cen- 
tral orb,  and  obtain  the  spectrum  of  the  gaseous  envelope, 
we  should  obtain  a  striped  spectrum,  each  bright  band  of 
which  would  coincide  with  one  of  Fraunhofer's  dark  lines. 
These  lines,  therefore,  are  spaces  of  relative,  not  of  abso- 
lute darkness  ;  upon  them  the  rays  of  the  absorbent  photo- 
sphere fall ;  but,  these  not  being  sufficiently  intense  to  make 
good  the  light  intercepted,  the  spaces  which  they  illuminate 
are  dark,  in  comparison  to  the  general  brilliancy  of  the 
spectrum. 

It  has  long  been  supposed  that  sun  and  planets  have 
had  a  common  origin,  and  that  hence  the  same  substances 
are  more  or  less  common  to  them  all.  Can  wre  detect  the 
presence  of  any  of  our  terrestrial  substances  in  the  sun  ?  I 
have  said  that  the  bright  bands  of  a  metal  are  character- 


I 
486  LECTURE    XIII. 

istic  of  the  metal ;  that  we  can,  without  seeing  the  metal, 
declare  its  name  from  the  inspection  of  the  bands.  The 
bands  are,  so  to  speak,  the  voice  of  the  metal  declaring  its 
presence.  Hence,  if  any  of  our  terrestrial  metals  be  con- 
tained in  the  sun's  atmosphere,  the  dark  lines  which  they 
produce  ought  to  coincide  exactly  with  the  bright  lines 
emitted  by  the  vapour  of  the  metal  itself.  In  the  case  of 
the  single  metal  iron,  about  60  bright  lines  have  been  de- 
termined as  belonging  to  it.  When  the  light  from  the 
incandescent  vapour  of  iron,  obtained  by  passing  elec- 
tric sparks  between  two  iron  wires,  is  allowed  to  pass 
through  one-half  of  a  fine  slit,  and  the  light  of  the  sun 
through  the  other  half,  the  spectra  from  both  sources  of 
light  may  be  placed  together ;  and  when  this  is  done  it  is 
found  that  for  every  bright  line  of  the  iron  spectrum  there 
is  a  dark  line  of  the  solar  spectrum.  Reduced  to  actual 
calculation,  this  means  that  the  chances  are  more  than  1,000,- 
000,000,000,000,000,  to  1  that  iron  is  in  the  atmosphere  of 
the  sun.  Comparing  the  spectra  of  other  metals  in  the 
same  manner,  Professor  Kirchhof,  to  whose  genius  we  owe 
this  splendid  generalisation,  finds  iron,  calcium,  magnesium, 
sodium,  chromium,  and  other  metals,  to  be  constituents  of 
the  solar  atmosphere,  but  as  yet  he  has  been  unable  to  de- 
tect gold,  silver,  mercury,  aluminium,  tin,  lead,  arsenic,  or 
antimony. 

I  can  imitate  in  a  way  more  precise  than  that  hitherto 
employed,  the  solar  constitution  here  supposed.  I  place  in 
the  electric  lamp  a  cylinder  of  carbon  about  half  an  inch 
thick ;  on  the  top,  and  round  about  the  edge  of  the  cylin- 
der, I  place  a  ring  of  sodium,  leaving  the  central  portion 
of  the  cylinder  clear.  I  bring  down  the  upper  coal  point 
upon  the  middle  of  the  cylinder's  upper  surface,  thus  pro- 
ducing the  ordinary  electric  light.  The  proximity  of  this 
light  to  the  sodium  is  sufficient  to  volatilise  the  latter,  and 
thus  I  surround  my  little  central  sun  with  an  atmosphere 


SOLAR  EMISSION. 


487 


Fig.  100. 


of  sodium  vapour,  as  the  real  sun  is  surrounded  by  its 
photosphere.     In  the  spectrum  of  this  light  you  see  the  yel- 
low band  is  absent. 

The  energy  of  solar  emission 
has  been  measured  by  Sir  John 
Herschcl  at  the  Cape  of  Good 
Hope,  and  by  M.  Pouillet  in 
Paris.  The  agreement  between 
the  measurements  is  very  re- 
markable. Sir  John  Herschel 
finds  the  direct  heating  effect  of 
a  vertical  sun,  at  the  sea  level,  to 
be  competent  to  melt  0*00754  of 
an  inch  of  ice  per  minute  ;  while 
according  to  M.  .  Pouillet,  the 
quantity  is  0'00703  of  an  inch. 
The  mean  of  the  determinations 
cannot  be  far  from  the  truth; 
this  gives  0-00728  of  an  inch  of 
ice  per  minute,  or  nearly  half  an 
inch  per  hour.  Before  you  (fig. 
100)  I  have  placed  an  instrument 
similar  in  form  to  that  used  by 
M.  Pouillet,  and  called  by  him  a 
pyrheliometer.  The  particular  in- 
strument which  you  now  see  is  composed  of  a  shallow  cylin- 
der of  steel,  a  a,  which  is  filled  with  mercury.  Into  the  cylin- 
der this  thermometer  c?,  is  introduced,  the  stem  of  which  is 
protected  by  a  piece  of  brass  tubing.  We  thus  obtain  the 
temperature  of  the  mercury.  The  flat  end  of  the  cylinder 
is  to  be  turned  towards  the  sun,  and  the  surface  thus  pre- 
sented is  coated  with  lampblack.  Here  is  a  collar  and 
screw,  c  c,  by  means  of  which  the  instrument  may  be  at- 
tached to  the  stake  driven  into  the  ground,  or  into  the 
snow,  if  the  observations  are  made  at  considerable  heights. 


4:88  LECTUEE    XIII. 

It  is  necessary  that  the  surface  which  receives  the  sun's 
rays  should  be  perpendicular  to  the  rays,  and  this  is  secured 
by  appending  to  the  brass  tube  which  shields  the  stem  of 
the  thermometer,  a  disk,  e  e,  of  precisely  the  same  diameter 
as  the  steel  cylinder.  When  the  shadow  of  the  cylinder 
accurately  covers  the  disk,  we  are  sure  that  the  rays  fall, 
as  perpendiculars,  on  the  upturned  surface  of  the  cylinder. 
The  observations  are  made  in  the  following  manner : — 
First,  the  instrument  is  permitted,  not  to  receive  the  sun's 
rays,  but  to  radiate  its  own  heat  for  five  minutes  against 
an  unclouded  part  of  the  firmament ;  the  decrease  of  the 
temperature  of  the  mercury  consequent  on  this  radiation  is 
then  noted.  Next,  the  instrument  is  turned  towards  the 
sun,  so  that  the  solar  rays  fall  perpendicularly  upon  it  for 
five  minutes, — the  augmentation  of  temperature  is  noAv 
noted.  Finally,  the  instrument  is  turned  again  towards 
the  firmament,  away  from  the  sun,  and  allowed  to  radiate 
for  another  five  minutes,  the  sinking  of  the  thermometer 
being  noted  as  before.  You  might,  perhaps,  suppose  that 
exposure  to  the  sun  alone  would  be  sufficient  to  determine 
his  heating  power ;  but  we  must  not  forget  that  during 
the  whole  time  of  exposure  to  the  sun's  action,  the  black- 
ened surface  of  the  cylinder  is  also  radiating  into  space ; 
it  is  not  therefore  a  case  of  pure  gam :  the  heat  received 
from  the  sun  is,  in  part,  thus  wasted,  even  while  the  ex- 
periment is  going  on ;  and  to  find  the  quantity  lost,  the 
first  and  last  experiments  are  needed.  In  order  to  obtain 
the  whole  heating  power  of  the  sun,  we  must  add  to  his 
observed  heating  power,  the  quantity  lost  during  the  time 
of  exposure,  and  this  quantity  is  the  mean  of  the  first  and 
last  observations.  Supposing  the  letter  E  to  represent  the 
augmentation  of  temperature  by  five  minutes'  exposure  to 
the  sun,  and  that  t  and  t'  represent  the  reductions  of  tem- 
perature observed  before  and  after,  then  the  whole  force 
of  the  sun,  which  we  may  call  T,  would  be  thus  expressed : 


INFLUENCE   OF  THE   EABTH's   ATMOSPHERE.  489 


= 

The  surftice  on  which  the  sun's  rays  here  fall  is  known  ; 
the  quantity  of  mercury  within  the  cylinder  is  also  known  ; 
hence  we  can  express  the  effect  of  the  sun's  heat  upon  a 
given  area,  by  stating  that  it  is  competent,  in  five  minutes, 
to  raise  so  much  mercury,  or  so  much  water,  so  many  de- 
grees in  temperature.  Water  indeed,  instead  of  mercury, 
was  used  in  M.  Pouillet's  pyrheliometer. 

The  observations  were  made  at  different  hours  of  the 
day,  and,  hence,  through  different  thicknesses  of  the  earth's 
atmosphere  ;  augmenting  from  the  minimum  thickness  at 
noon,  up  ot  the  maximum  at  6  P.  M.,  which  was  the  time  of 
the  latest  observation.  It  was  found  that  the  solar  energy 
diminished  according  to  a  certain  law,  as  the  thickness 
of  the  air  crossed  by  the  sunbeams  increased  ;  and  from 
this  law  M.  Pouillet  was  enabled  to  infer  what  the  atmos- 
pheric absorption  of  a  beam  would  be,  if  directed  down- 
wards to  his  instrument  from  the  zenith.  This  he  found 
to  be  25  per  cent.  Doubtless,  this  absorption  would  be 
chiefly  exerted  upon  the  longer  undulations  emitted  by  the 
sun,  the  aqueous  vapour  of  our  air,  and  not  the  air 
itself,  being  the  main  agent  of  absorption.  Taking  into 
account  the  whole  terrestrial  hemisphere  turned  towards 
the  sun,  the  amount  intercepted  by  the  atmospheric  en- 
velope is  four-tenths  of  the  entire  radiation  in  the  direction 
of  the  earth.  Thus,  were  the  atmosphere  removed,  the 
illuminated  hemisphere  of  the  earth  would  receive  nearly 
twice  the  amount  of  heat  from  the  sun  that  now  reaches 
it.  The  total  amount  of  solar  heat  received  by  the  earth 
in  a  year,  if  distributed  uniformly  over  the  earth's  surface, 
would  be  sufficient  to  liquefy  a  layer  of  ice  100  feet  thick, 
and  covering  the  whole  earth. 

Knowing  thus  the  annual  receipt  of  the  earth,  we  can 
calculate  the  entire  quantity  of  heat  emitted  by  the  sun  in 
21* 


490  LECTURE    XIII. 

a  year.  Conceive  a  hollow  sphere  to  surround  the  sun,  its 
centre  being  the  sun's  centre,  and  its  surface  at  the  dis- 
tance of  the  earth  from  the  sun.  The  section  of  the  earth 
cut  by  this  surface,  is  to  the  whole  area  of  the  hollow 
sphere,  as  1  :  2,300,000,000  ;  hence,  the  quantity  of  solar 
heat  intercepted  by  the  earth  is  only  ^^ oVoooir  of  the 
total  radiation. 

The  heat  emitted  by  the  sun,  if  used  to  melt  a  stratum 
of  ice  applied  to  the  sun's  surface,  would  liquefy  the 
ice  at  the  rate  of  2,400  feet  an  hour.  It  would  boil,  per 
hour,  700,000  millions  of  cubic  miles  of  ice-cold  water. 
Expressed  in  another  form,  the  heat  given  out  by  the  sun, 
per  hour,  is  equal  to  that  which  would  be  generated  by  the 
combustion  of  a  layer  of  solid  coal,  10  feet  thick,  entirely 
surrounding  the  sun  ;  hence,  the  heat  emitted  in  a  year  is 
equal  to  that  which  would  be  produced  by  the  combustion 
of  a  layer  of  coal  17  miles  in  thickness. 

These  are  the  results  of  direct  measurement ;  and 
should  greater  accuracy  be  conferred  on  them  by  future 
determinations,  it  will  not  deprive  them  of  their  astound- 
ing character.  And  this  expenditure  has  been  going  on 
for  ages,  without  our  being  able,  in  historic  times,  to  de- 
tect the  loss.  When  the  tolling  of  a  bell  is  heard  at  a  dis- 
tance, the  sound  of  each  stroke  soon  sinks,  the  sonorous 
vibrations  are  quickly  wasted,  and  renewed  strokes  are 
necessary  to  maintain  the  sound.  Like  the  bell, 

Die  Sonne  tont  nach  alter  Weise. 

But  how  is  its  tone  sustained  ?  How  is  the  perennial 
loss  of  the  sun  made  good  ?  We  are  apt  to  overlook  the 
wonderful  in  the  common.  Possibly  to  many  of  us — and 
even  to  some  of  the  most  enlightened  among  us — the  sun 
appears  as  a  fire,  differing  from  our  terrestrial  fires  only  in 
the  magnitude  and  intensity  of  its  combustion.  But  what 
is  the  burning  matter  which  can  thus  maintain  itself?  AH 


MAINTENANCE  OF  60LAB  POWER.  491 

that  we  know  of  cosmical  phenomena  declares  our  brother- 
hood with  the  sun, — affirms  that  the  same  constituents 
enter  into  the  composition  of  his  mass  as  those  already 
known  to  chemistry.  But  no  earthly  substance  with  which 
we  are  acquainted — no  substance  which  the  fall  of  meteors 
has  landed  on  the  earth — would  be  at  all  competent  to. 
maintain  the  sun's  combustion.  The  chemical  energy  of 
such  substances  would  be  too  weak,  and  their  dissipation 
would  be  too  speedy.  "Were  the  sun  a  solid  block  of  coal, 
and  were  it  allowed  a  sufficient  supply  of  oyygen,  to  enable 
it  to  burn  at  the  rate  necessary  to  produce  the  observed 
emission,  it  would  be  utterly  consumed  in  5,000  years. 
On  the  other  hand,  to  imagine  it  a  body  originally  en- 
dowed with  a  store  of  heat — a  hot  globe  now  cooling — 
necessitates  the  ascription  to  it  of  qualities,  wholly  differ- 
ent from  those  possessed  by  terrestrial  matter.  If  we  knew 
the  specific  heat  of  the  sun,  we  could  calculate  its  rate  of 
cooling.  Assuming  this  to  be  the  same  as  that  of  water — 
the  terrestrial  substance  which  possesses  the  highest  spe- 
cific heat — at  its  present  rate  of  emission,  the  entire  mass 
of  the  sun  would  cool  down  15,000°  Faht.  in- 5,000  years. 
In  short,  if  the  sun  be  formed  of  matter  like  our  own, 
some  means  must  exist  of  restoring  to  him  his  wasted 
power. 

The  facts  are  so  extraordinary,  that  the  soberest  hy- 
pothesis regarding  them  must  appear  wild.  The  sun  we 
know  rotates  upon  his  axis  ;  he  turns  like  a  wheel  once  in 
about  25  days:  can  it  be  the  friction  of  the  periphery 
of  this  wheel  against  something  in  surrounding  space 
which  produces  the  light  and  heat  ?  Such  a  notion  has 
been  entertained.  But  what  forms  the  brake,  and  by  what 
agency  is  it  held,  while  it  rubs  against  the  sun  ?  The  ac- 
tion is  inconceivable ;  but,  granting  the  existence  of  the 
brake,  we  can  calculate  the  total  amount  of  heat  which  the 
sun  could  generate  by  such  friction.  We  know  his  mass, 


4:92  LECTUKE    XHI. 

we  know  his  time  of  rotation ;  we  know  the  mechanical 
equivalent  of  heat  ;  and  from  these  data  we  deduce,  with 
certainty,  that  the  entire  force  of  rotation,  if  converted  into 
heat,  would  cover  more  than  one,  but  less  than  two  cen- 
turies of  emission.*  There  is  no  hypothesis  involved  in 
this  calculation. 

There  is  another  theory,  which,  however  bold  it  may,  at 
first  sight,  appear,  deserves  our  earnest  attention.  I  have 
already  referred  to  it  as  the  Meteoric  Theory  of  the  sun's 
heat.  Solar  space  is  peopled  with  ponderable  objects: 
Kepler's  celebrated  statement  that '  there  are  more  comets 
in  the  heavens  than  fish  in  the  ocean,'  refers  to  the  fact 
that  a  small  portion  only  of  the  total  number  of  comets  be- 
longing to  our  system,  are  seen  from  the  earth.  But 
besides  comets,  and  planets,  and  moons,  a  numerous  class 
of  bodies  belong  to  our  system, — asteroids,  which,  from 
their  smallness,  might  be  regarded  as  cosmical  atoms.  Like 
the  planets  and  the  comets  these  smaller  bodies  obey  the 
law  of  gravity,  and  revolve  on  elliptic  orbits  round  the  sun  ; 
and  it  is  they,  when  they  come  within  the  earth's  atmo- 
sphere, that,  fired  by  friction,  appear  to  us  as  meteors  and 
falling  stars. 

On  a  bright  night,  20  minutes  rarely  pass  at  any  part 
of  the  earth's  surface  without  the  appearance  of  at  least 
one  meteor.  At  certain  times  (the  12th  of  August  and  the 
14th  of  November)  they  appear  in  enormous  numbers. 
During  nine  hours  of  observation  in  Boston,  when  they  were 
described  as  falling  as  thick  as  snowflakes,  240,000  meteors 
were  calculated  to  have  been  observed.  The  number  fall- 
ing in  a  year  might,  perhaps,  be  estimated  at  hundreds  or 
'  ousands  of  millions,  and  even  these  would  constitute  but 
small  portion  of  the  total  crowd  of  asteroids  that  circu- 
late round  the  sun.  From  the  phenomena  of  light  and 

*  Meyer  Dynamik  des  Himmels,  p.  10. 


THE   ZODIACAL   LIGHT.  493 

heat,  and  by  the  direct  observations  of  Encke  on  his  comet, 
we  learn  that  the  universe  is  filled  by  a  resisting  medium, 
through  the  friction  of  which  all  the  masses  of  our  system 
are  drawn  gradually  towards  the  sun.  And  though  the 
larger  planets  show,  in  historic  tunes,  no  diminution  of 
their  periods  of  revolution,  this  may  not  hold  good  for  the 
smaller  bodies.  In  the  time  required  for  the  mean  distance 
of  the  earth  from  the  sun  to  alter  a  single  yard,  a  small 
asteroid  may  have  approached  thousands  of  miles  nearer  to 
our  central  luminary. 

Following  up  these  reflections  we  should  infer,  that 
while  this  immeasurable  stream  of  ponderable  matter  rolls 
unceasingly  towards  the  sun,  it  must  augment  in  density 
as  it  approaches  its  centre  of  convergence.  And  here  the 
conjecture  naturally  rises,  that  that  weak  nebulous  light, 
of  vast  dimensions,  which  embraces  the  sun — the  Zodiacal 
Light — may  owe  its  existence  to  these  crowded  meteoric 
masses.  However  this  may  be,  it  is  at  least  proved  that 
this  luminous  phenomenon  arises  from  matter  which  cir- 
culates in  obedience  to  planetary  laws ;  the  entire  mass 
constituting  the  zodiacal  light  must  be  constantly  ap- 
proaching, and  incessantly  raining  its  substance  down 
upon  the  sun. 

We  observe  the  fall  of  an  apple  and  investigate  the  law 
which  rules  its  motion.  In  the  place  of  the  earth  we  set 
the  sun,  and  in  the  place  of  the  apple  we  set  the  earth,  and 
thus  possess  ourselves  of  the  key  to  the  mechanics  of  the 
heavens.  We  now  know  the  connection  between  height 
of  fall,  velocity,  and  heat  of  the  surface  of  the  earth.  In 
the  place  of  the  earth  let  us  set  the  sun,  with  300,000  times 
the  earth's  mass,  and,  instead  of  a  fall  of  a  few  feet,  let  us 
take  cosmical  elevations ;  we  thus  obtain  a  means  of  gen- 
erating heat  which  transcends  all  terrestrial  power. 

It  is  easy  to  calculate  both  the  maximum  and  the  mini- 
mum velocity,  imparted  by  the  sun's  attraction  to  an  as- 


4:94:  LECTCKE    XIII. 

teroid  circulating  round  him ;  the  maximum  is  generated 
when  the  body  approaches  the  sun  from  an  infinite  dis- 
tance ;  the  entire  pull  of  the  sun  being  then  expended  upon 
it ;  the  minimum  is  that  velocity  which  w^ould  barely  en- 
able the  body  to  revolve  round  the  sun  close  to  his  surface. 
The  final  velocity  of  the  former,  just  before  striking  the 
sun,  would  be  390  miles  a  second,  that  of  the  latter  276 
miles  a  second.  The  asteroid,  on  striking  the  sun  with  the 
former  velocity,  would  develope  more  than  9,000  times  the 
heat  generated  by  the  combustion  of  an  equal  asteroid  of 
solid  coal ;  while  the  shock,  in  the  latter  case,  would  gen- 
erate heat  equal  to  that  of  the  combustion  of  upwards  of 
4,000  such  asteroids.  It  matters  not,  therefore,  whether 
the  substances  falling  into  the  sun  be  combustible  or  not ; 
their  being  combustible  would  not  add  sensibly  to  the  tre- 
mendous heat  produced  by  their  mechanical  collision. 

Here  then  we  have  an  agency  competent  to  restore  his 
lost  energy  to  the  sun,  and  to  maintain  a  temperature  at 
his  surface  which  transcends  all  terrestrial  combustion. 
The  very  quality  of  the  solar  rays — their  incomparable 
penetrative  power — enables  us  to  infer  that  the  tempera- 
ture of  their  origin  must  be  enormous  ;  but  in  the  fall  of 
asteroids  we  find  the  means  of  producing  such  a  tempera- 
ture. It  may  be  contended  that  this  showering  down  of 
matter  must  be  accompanied  by  the  growth  of  the  sun  in 
size  ;  it  is  so ;  but  the  quantity  necessary  to  produce  the 
observed  calorific  emission,  even  if  accumulated  for  4,000 
years,  would  defeat  the  scrutiny  of  our  best  instruments. 
If  the  earth  struck  the  sun  it  would  utterly  vanish  from 
perception,  but  the  heat  developed  by  its  shock  would 
cover  the  expenditure  of  the  sun  for  a  century. 

To  the  earth  itself  apply  considerations  similar  to  those 
which  we  have  applied  to  the  sun.  Newton's  theory  of 
gravitation,  which  enables  us,  from  the  present  form  of 
the  earth,  to  deduce  its  original  state  of  aggregation,  re- 


THE   TIDES    AND   THE    EARTH'S    ROTATION.  495 

veals  to  us,  at  the  same  time,  a  source  of  heat  powerful 
enough  to  bring  about  the  fluid  state — powerful  enough  to 
fuse  even  worlds.  It  teaches  us  to  regard  the  molten  con- 
dition of  a  planet  as  resulting  from  the  mechanical  union 
of  cosmical  masses,  and  thus  reduces  to  the  same  homo- 
geneous process,  the  heat  stored  up  in  the  body  of  the 
earth,  and  the  heat  emitted  by  the  sun. 

Without  doubt  the  whole  surface  of  the  sun  displays  an 
unbroken  ocean  of  fiery  fluid  matter.  On  this  ocean  rests 
an  atmosphere  of  glowing  gas — a  flame  atmosphere,  or 
photosphere.  But  gaseous  substances,  when  compared  with 
solid  ones,  emit,  even  when  their  temperature  is  very  high, 
only  a  feeble  and  transparent  light.  Hence  it  is  probable 
that  the  dazzling  white  light  of  the  sun  comes  through  the 
atmosphere,  from  the  more  solid  portions  of  the  surface.* 

There  is  one  other  consideration  connected  with  the 
permanence  of  our  present  terrestrial  conditions,  which  is 
well  worthy  of  our  attention.  Standing  upon  one  of  the 
London  bridges,  we  observe  the  current  of  the  Thames  re- 
versed, and  the  water  poured  upwards  twice  a-day.  The 
water  thus  moved  rubs  against  the  river's  bed  and  sides, 
and  heat  is  the  consequence  of  this  friction.  The  heat  thus 
generated  is,  in  part,  radiated  into  space,  and  there  lost,  as 
far  as  the  earth  is  concerned.  What  is  it  that  supplies 
this  incessant  loss  ?  The  earth's  rotation.  Let  us  look  a 
little  more  closely  at  this  matter.  Imagine  the  moon  fixed, 
and  the  earth  turning  like  a  wheel  from  west  to  east  in  its 
diurnal  rotation.  A  mountain  on  the  earth's  surface,  on 
approaching  the  moon's  meridian,  is,  as  it  were,  laid  hold 
of  by  the  moon ;  forms  a  kind  of  handle  by  which  the 
earth  is  pulled  more  quickly  round.  But  when  the  meridian 
is  passed  the  pull  of  the  moon  on  the  mountain  would  be 

*  I  am  quoting  here  from  Mayer,  but  this  is  the  exact  view  now  enter- 
tained by  Kirchhof.  We  sec  the  solid  or  liquid  mass  of  the  sun  through 
his  photosphere. 


496  LECTUKE  xm. 

in  the  opposite  direction ;  it  now  tends  to  diminish  the 
velocity  of  rotation  as  much  as  it  previously  augmented  it ; 
and  thus  the  action  of  all  fixed  bodies  on  the  earth's  sur- 
face is  neutralised. 

But  suppose  the  mountain  to  lie  always  to  the  east  of 
the  moon's  meridian,  the  pull  then  would  be  always  exert- 
ed against  the  earth's  rotation,  the  velocity  of  which  would 
be  diminished  in  a  degree  corresponding  to  the  strength  of 
the  pull.  TJie  tidal  wave  occupies  this  position — it  lies  al- 
ways to  the  east  of  the  moon's  meridian  ;  the  waters  of  the 
ocean  are,  in  part,  dragged  as  a  brake  along  the  surface  of 
the  earth,  and  as  a  brake  they  must  diminish  the  velocity 
of  the  earth's  rotation.  The  diminution,  though  inevitable, 
is,  however,  too  small  to  make  itself  felt  within  the  period 
over  which  observations  on  the  subject  extend.  Suppos- 
ing, then,  that  we  turn  a  mill  by  the  action  of  the  tide,  and 
produce  heat  by  the  friction  of  the  millstones ;  that  heat 
has  an  origin  totally  different  from  the  heat  produced  by 
another  pair  of  millstones  which  are  turned  by  a  mountain 
stream.  The  former  is  produced  at  the  expense  of  the 
earth's  rotation  ;  the  latter  at  the  expense  of  the  sun's  ra- 
diation, which  lifted  the  millstream  to  its  source.* 

Such  is  an  outline  of  the  Meteoric  Theory  of  the  sun's 
heat,  as  extracted  from  Mayer's  Essay  on  Celestial  Dynam- 
ics. I  have  held  closely  to  his  statements,  and  in  most 
cases  simply  translated  his  words.  But  the  sketch  conveys 
no  adequate  idea  of  the  firmness  and  consistency  with 
which  he  has  applied  his  principles.  He  deals  with  true 
causes  ;  and  the  only  question  that  can  affect  his  theory  re- 
fers to  the  quantity  of  action  which  he  has  ascribed  to  these 
causes.  I  do  not  pledge  myself  to  this  theory,  nor  do  I  ask 
you  to  accept  it  as  demonstrated  ;  still  it  would  be  a  great 
mistake  to  regard  it  as  chimerical.  It  is  a  noble  specula- 

*  Dynamik  des  Himmels  p.  38,  &c. 


DYNAMIC   RADIATION.       ^4,^  497" 

tion  ;  and  depend  upon  it,  the  true  theory,  if  this,  or  some 
form  of  it,  be  not  the  true  one,  will  not  appear  less  wild  or 
less  astounding.* 

Mayer  published  his  Essay  in  1848  ;  five  years  after- 
wards Mr.  Waterston  sketched,  independently,  a  similar 
theory,  at  the  Hull  Meeting  of  the  British  Association. 
The  Transactions  of  the  Royal  Society  of  Edinburgh  for 
1854  contain  an  extremely  beautiful  memoir,  by  Professor 
William  Thomson,  in  which  Mr.  Waterston's  sketch  is  de- 
veloped. He  considers  that  the  meteors  which  are  to  fur- 
nish stores  of  energy  for  our  future  sunlight,  lie  principally 
within  the  earth's  orbit,  and  that  we  see  them  there,  as  the 
Zodiacal  Light,  '  an  illuminated  shower,  or  rather  tornado, 
of  stones '  (Herschel,  §  897).  Thus  he  points  to  the  precise 
source  of  power  previously  indicated  by  Mayer.  '  In  con- 
clusion, then,'  writes  Professor  Thomson,  '  the  source  of 
energy  from  which  solar  heat  is  derived  is  undoubtedly 
meteoric.  .  .  .  The  principal  source — perhaps  the  sole  ap- 
preciable efficient  source — is  in  bodies  circulating  round  the 

*  While  preparing  these  sheets  finally  for  press,  I  had  occasion  to  look 
once  more  into  the  writings  of  Mayer,  and  the  effect  was  a  revival  of  the 
interest  with  which  I  first  read  them.  Dr.  Mayer  was  a  working  physician 
in  the  little  German  town  of  Heilbronn,  who,  in  1840,  made  the  observation 
that  the  venous  blood  of  a  feverish  patient  in  the  tropics  was  redder  than 
in  more  northern  latitudes.  Starting  from  this  fact,  while  engaged  in  the 
duties  of  a  laborious  profession,  and  apparently  without  a  single  kindred 
spirit  to  support  and  animate  him,  Mayer  raised  his  mind  to  the  level 
indicated  by  the  references  made  to  his  works,  throughout  this  book.  In 
1842  he  published  his  first  memoir  '  On  the  Forces  of  Inorganic  Nature  ; ' 
in  1845,  his  '  Organic  Motion'  was  published  ;  and  in  1848,  his  '  Celestial 
D}Tnamics'  appeared.  After  this,  his  overtasked  brain  gave  way,  and  a 
cloud  settled  on  the  intellect  which  had  accomplished  so  much.  The  shade 
however,  was  but  temporary,  and  Dr.  Mayer  is  now  restored.  I  have  never 
seen  him,  nor  has  a  line  of  correspondence  ever  passed  between  us. 
Modestly  and  noiselessly  he  has  done  his  work  ;  and  having  spoken  of  his 
merits,  as  accident  made  it  my  duty  to  speak,  I  confidently  leave  to  history 
the  care  of  his  fame. 


498  LECTUKE   XIII. 

sun  at  present  inside  the  earth's  orbit,  and  probably  seen 
in  the  sunlight  by  us  called  "  Zodiacal  Light."  The  store 
of  energy  for  future  sunlight  is  at  present  partly  dynamical 
— that  of  the  motions  of  these  bodies  round  the  sun ;  and 
partly  potential — that  of  their  gravitation  towards  the  sun. 
This  latter  is  gradually  being  spent,  half  against  the  resist- 
ing medium,  and  half  in  causing  a  continuous  increase  of 
the  former.  Each  meteor  thus  goes  on  moving  faster  and 
faster,  and  getting  nearer  and  nearer  the  centre,  until  some 
time,  very  suddenly,  it  gets  so  much  entangled  in  the  solar 
atmosphere  as  to  begin  to  lose  velocity.  In  a  few  seconds 
more  it  is  at  rest  on  the  sun's  surface,  and  the  energy  given 
up  is  vibrated  across  the  district  where  it  was  gathered 
during  so  many  ages,  ultimately  to  penetrate,  as  light,  the 
remotest  regions  of  space.' 

From  the  tables  published  by  Prof.  Thomson  I  extract 
the  following  interesting  data ;  firstly,  with  reference  to 
the  amount  of  heat  equivalent  to  the  rotation  of  the  sun 
and  planets  round  their  axes ;  the  amount,  that  is,  which 
would  be  generated,  supposing  a  brake  applied  at  the  sur- 
faces of  the  sun  and  planets,  until  the  motion  of  rotation 
was  entirely  stopped :  secondly,  with  reference  to  the 
amount  of  heat  due  to  the  sun's  gravitation — the  heat, 
that  is,  which  would  be  developed  by  each  of  the  planets 
in  falling  into  the  sun.  The  quantity  of  heat  is  expressed  in 
terms  of  the  time  during  which  it  would  cover  the  solar 
en>:-ssion. 

Heat  of  Gravitation,  equal  to  Solar  Heat  of  Rotation,  equal  to  Solar 

emission  for  a  period  of  emission  for  a  period  of 

Sun        .  .  ,  .  .116  years      6  days 

Mercury  .         6  years  214  days    .  15     „ 

99     „ 
81 
7 

14     „     144 
2     „     127 
71 


Venus    . 

.       83 

227 

Earth      . 

94 

303 

Mars       . 

.       12 

252 

Jupiter  . 
Saturn    . 

32240 
.  9650 

• 

Uranus  . 

.  1610 

. 

Neptune 

.  1890 

. 

ENERGIES   OF   THE   SOLAK   SYSTEM.  490 

Thus,  if  the  planet  Mercury  were  to  strike  the  sun,  the 
quantity  of  heat  generated  would  cover  the  solar  emission 
for  nearly  seven  years ;  while  the  shock  of  Jupiter  would 
cover  the  loss  of  32,240  years.  Our  earth,  would  furnish 
a  supply  for  95  years.  The  heat  of  rotation  of  the  sun 
and  planets,  taken  together,  would  cover  the  solar  emis- 
sion for  134  years;  while  the  total  heat  of  gravitation 
(that  produced  by  the  planets  falling  into  the  sun)  would 
cover  the  emission  for  45,589  years. 

Whatever  be  the  ultimate  fate  of  the  theory  here 
sketched,  it  is  a  great  thing  to  be  able  to  state  the  condi- 
tions which  certainly  would  produce  a  sun, — to  be  able  to 
discern  in  the  force  of  gravity,  acting  upon  dark  matter, 
the  source  from  which  the  starry  heavens  may  have  been 
derived.  For,  whether  the  sun  be  produced  and  his  emis- 
sion maintained  by  the  collision  of  cosmical  masses, — 
whether  the  internal  heat  of  the  earth  be  the  residue  of 
that  developed  by  the  impact  of  cold  dark  asteroids,  or 
not,  there  cannot  be  a  doubt  as  to  the  competence  of  the 
cause  assigned  to  produce  the  effects  ascribed  to  it.  Solar 
light  and  solar  heat  lie  latent  in  the  force  which  pulls  an 
apple  to  the  ground.  '  Created  simply  as  a  difference  of 
position  of  attracting  masses,  the  potential  energy  of  grav- 
itation was  the  original  form  of  all  the  energy  in  the  uni- 
verse. As  surely  as  the  weights  of  a  clock  run  down  to 
their  lowest  position,  from  which  they  can  never  rise  again 
unless  fresh  energy  is  communicated  to  them  from  some 
source  not  yet  exhausted,  so  surely  must  planet  after 
planet  creep  in,  age  by  age,  towards  the  sun.  When  each 
comes  within  a  few  hundred  thousand  miles  of  his  surface, 
if  he  is  still  incandescent,  it  must  be  melted  and  driven 
into  vapour  by  radiant  heat.  Nor,  if  he  be  crusted  over 
and  become  dark  and  cool  externally,  can  the  doomed 
planet  escape  its  fiery  end.  If  it  does  not  become  incan- 
descent, like  a  shooting  star,  by  friction  in  its  passage 


500  LECTURE  xin. 

through  his  atmosphere,  its  first  graze  on  his  surface  must 
produce  a  stupendous  flash  of  light  and  heat.  It  may  be 
at  once,  or  it  may  be  after  two  or  three  bounds  like  a  can- 
non-shot ricochetting  on  a  surface  of  earth  or  water,  the 
whole  mass  must  be  crushed,  melted,  and  evaporated  by  a 
crash,  generating  in  a  moment  some  thousands  of  times  as 
much  heat  as  a  coal  of  the  same  size  would  produce  by 
burning.* 

Helmholtz,  an  eminent  German  physiologist,  physicist, 
and  mathematician,  takes  a  somewhat  different  view  of  the 
origin  and  maintenance  of  solar  light  and  heat.  He  starts 
from  the  nebular  hypothesis  of  Laplace,  and  assuming  the 
nebulous  matter,  in  the  first  instance,  to  have  been  of  ex- 
treme tenuity,  he  determines  the  amount  of  heat  gener- 
ated by  its  condensation  to  the  present  solar  system.  Sup- 
posing the  specific  heat  of  the  condensing  mass  to  be  the 
same  as  that  of  water,  then  the  heat  of  condensation 
would  be  sufficient  to  raise  their  temperature  28,000,000° 
Centigrade.  By  far  the  greater  part  of  this  heat  was 
wasted,  ages  ago,  in  space.  The  most  intense  terrestrial 
combustion  that  we  can  command  is  that  of  oxygen  and 
hydrogen,  and  the  temperature  of  the  pure  oxyhydrogen 
flame  is  8,061°  C.  The  temperature  of  a  hydrogen  flame, 
burning  in  air,  is  3,259°  C. ;  while  that  of  the  lime  light, 
which  shines  with  such  sunlike  brilliancy,  is  estimated  at 
2,000°  C.  What  conception,  then,  can  we  form  of  a  tem- 
perature more  than  thirteen  thousand  times  that  of  the 
Drummond  light  ?  If  our  system  were  composed  of  pure 
coal,  and  burnt  up,  the  heat  produced  by  its  combustion 
would  only  amount  to  -g-gVirth  of  that  generated  by  the 
condensation  of  the  nebulous  matter,  to  form  our  solar 
system.  Helmholtz  supposes  this  condensation  to  con- 
tinue ;  that  a  virtual  falling  down  of  the  superficial  portions 

*  Thomson  and  Tait  in  « Good  Words,'  Oct.  1862,  p.  606. 


501 


of  the  sun  towards  the  centre  still  takes  place,  a  continual 
development  of  heat  being  the  result.  However  this  may 
be,  he  shows  by  calculation  that  the  shrinking  of  the  sun's 
diameter  by  y^J^th  of  its  present  length,  would  generate 
an  amount  of  heat  competent  to  cover  the  solar  emission 
for  2,000  years ;  while  the  shrinking  of  the  sun  from  its 
present  mean  density  to  that  of  the  earth,  would  have  its 
equivalent  in  an  amount  of  heat  competent  to  cover  the 
present  solar  emission  for  17,000,000  of  years. 

'  But,'  continues  Helmholtz,  '  though  the  store  of  our 
planetary  system  is  so  immense  that  it  has  not  been  sensi- 
bly diminished  by  the  incessant  emission  which  has  gone 
on  during  the  period  of  man's  history,  and  though  the 
time  which  must  elapse  before  a  sensible  change  in  the 
condition  of  our  planetary  system  can  occur,  is  totally  be- 
yond our  comprehension,  the  inexorable  laws  of  mechan- 
ics show  that  this  store,  which  can  only  suffer  loss,  and 
not  gain,  must  finally  be  exhausted.  Shall  we  terrify  our- 
selves by  this  thought  ?  We  are  in  the  habit  of  measur- 
ing the  greatness  of  the  universe,  and  the  wisdom  dis- 
played in  it,  by  the  duration  and  the  profit  which  it  prom- 
ises to  our  own  race ;  but  the  past  history  of  the  earth 
shows  the  insignificance  of  the  interval  during  which  man 
has  had  his  dwelling  here.  What  the  museums  of  Europe 
show  us  of  the  remains  of  Egypt  and  Assyria  we  gaze 
upon  with  silent  wonder,  in  despair  of  being  able  to  carry 
back  our  thoughts  to  a  period  so  remote.  Still,  the  human 
race  must  have  existed  and  multiplied  for  ages  before  the 
Pyramids  could  have  been  erected.  We  estimate  the  du- 
ration of  human  history  at  6,000  years ;  but,  vast  as  this 
time  may  appear  to  us,  what  is  it  in  comparison  with  the 
period  during  which  the  earth  bore  successive  series  of 
rank  plants  and  mighty  animals,  but  no  men  ?  *  Periods 

*  The  absence  of  men  may  be  doubted.     See  Lubbock's  article  on  the 
'Antiquity  of  Man,'  in  the  'Natural  History  Review,'  July,  1862,  p.  267, 


502  LECTUEE   XIII. 

during  which,  in  our  own  neighbourhood  (Konigsberg), 
the  amber-tree  bloomed,  and  dropped  its  costly  gum  on 
the  earth  and  in  the  sea;  when  in  Europe  and  North 
America  groves  of  tropical  palms  flourished,  in  which  gi- 
gantic lizards,  and,  after  them,  elephants,  whose  mighty 
remains  are  still  buried  in  the  earth,  found  a  home. 
Different  geologists,  proceeding  from  different  premises, 
have  sought  to  estimate  the  length  of  the  above  period, 
and  they  set  it  down  from  one  to  nine  millions  of  years. 
The  time  during  which  the  earth  has  generated  organic 
beings  is  again  small,  compared  with  the  ages  during 
which  the  world  was  a  mass  of  molten  rocks.  The  ex- 
periments of  Bischof  upon  basalt  show  that  our  globe 
would  require  350  millions  of -years  to  cool  down  from 
2,000°  to  200°  Centigrade.  And  with  regard  to  the  period 
during  which  the  first  nebulous  masses  condensed,  to  form 
our  planetary  system,  conjecture  must  entirely  cease.  The 
history  of  man,  therefore,  is  but  a  minute  ripple  in  the 
infinite  ocean  of  time.  For  a  much  longer  period  than 
that  during  which  he  has  already  occupied  this  world,  the 
existence  of  a  state  of  inorganic  nature,  favourable  to 
man's  continuance  here,  seems  to  be  secured,  so  that  for 
ourselves,  and  for  long  generations  after  us,  we  have  noth- 
ing to  fear.  But  the  same  forces  of  air  and  water,  and  of 
the  volcanic  interior,  which  produced  former  geologic  rev- 
olutions, burying  one  series  of  living  forms  after  another, 
still  act  upon  the  earth's  crust.  They,  rather  than  those 
distant  cosmical  changes  of  which  we  have  spoken,  will 
put  an  end  to  the  human  race ;  and,  perhaps,  compel  us  to 
make  way  for  new  and  more  complete  forms  of  life,  as  the 
lizard  and  the  mammoth  have  given  way  to  us  and  our 
contemporaries.* 

With  reference  to  the  operations  of  the  sun  upon  the 

*  Wechselwirkung  der  Naturkrafte.     Phil.  Mag.,  Ser.  IV.  vol.  ix.  p.  515. 


HEKSCHEL   ON   SOLAB  INFLUENCE.  503 

earth,  its  ocean  and  its  atmosphere,  the  following  remark- 
able passage  was  written  by  Sir  John  Herschel  thirty-two 
years  ago.*  c  The  sun's  rays  are  the  ultimate  source  of 
almost  every  motion  which  takes  place  on  the  surface  of 
the  earth.  By  its  heat  are  produced  all  winds,  and  those 
disturbances  in  the  electric  equilibrium  of  the  atmosphere 
which  give  rise  to  the  phenomena  of  lightning,  and  prob- 
ably also  to  terrestrial  magnetism  and  the  Aurora.  By 
their  vivifying  action  vegetables  are  enabled  to  draw  sup- 
port from  inorganic  matter,  and  become  in  their  turn  the 
support  of  animals  and  man,  and  the  source  of  those  great 
deposits  of  dynamical  efficiency  which  are  laid  up  for  hu- 
man use  in  our  coal  strata.  By  them  the  waters  of  the  sea 
are  made  to  circulate  in  vapour  through  the  air,  and  irri- 
gate the  land,  producing  springs  and  rivers.  By  them  are 
produced  all  disturbances  of  the  chemical  equilibrium  of 
the  elements  of  nature,  which  by  a  series  of  compositions 
and  decompositions  give  rise  to  new  products  and  originate 
a  transfer  of  materials.  Even  the  slow  degradation  of  the 
solid  constituents  of  the  surface,  in  which  its  chief  geo- 
logical change  consists,  is  almost  entirely  due,  on  the  one 
hand,  to  the  abrasion  of  wind  or  rain  and  the  alternation 
of  heat  and  frost ;  on  the  other,  to  the  continual  beating 
of  sea  waves  agitated  by  winds,  the  results  of  solar  radia- 
tion. Tidal  action  (itself  partly  due  to  the  sun's  agency) 
exercises  here  a  comparatively  slight  influence.  The  effect 
of  oceanic  currents  (mainly  originating  in  that  influence), 
though  slight  in  abrasion,  is  powerful  in  diffusing  and 
transporting  the  matter  abraded ;  and  when  we  consider 
the  immense  transfer  of  matter  so  produced,  the  increase 
of  pressure  over  large  spaces  in  the  bed  of  the  ocean,  and 
diminution  over  corresponding  portions  of  the  land,  we 
are  riot  at  a  loss  to  perceive  how  the  elastic  force  of  sub- 

*  Outlines  of  Astronomy,  1833. 


504:  LECTUEE   XIII. 

terranean  fires,  thus  repressed  on  the  one  hand  and  released 
on  the  other,  may  break  forth  in  points  where  the  resist- 
ance is  barely  adequate  to  their  retention,  and  thus  bring 
the  phenomena  of  even  volcanic  activity  under  the  general 
law  of  solar  influence.' 

This  fine  passage  requires  but  the  breath  of  recent  inr 
vestigation  to  convert  it  into  an  exposition  of  the  law  of 
the  conservation  of  energy,  as  applied  to  both  the  organic 
and  inorganic  world.  Late  discoveries  have  taught  us 
that  winds  and  rivers  have  their  definite  thermal  values, 
and  that,  in  order  to  produce  their  motion,  an  equivalent 
amount  of  solar  heat  has  been  consumed.  While  they 
exist  as  winds  and  rivers,  the  heat  expended  in  producing 
them  has  ceased  to  exist  as  heat,  being  converted  into 
mechanical  motion ;  but  when  that  motion  is  arrested,  the 
heat  which  produced  it  is  restored.  A  river,  in  descending 
from  an  elevation  of  73'720  feet,  generates  an  amount  of 
heat  competent  to  augment  its  own  temperature  10°  Fahr., 
and  this  amount  of  heat  was  abstracted  from  the  sun,  in 
order  to  lift  the  matter  of  the  river  to  the  elevation  from 
which  it  falls.  As  long  as  the  river  continues  on  the 
heights,  whether  in  the  solid  form  as  a  glacier,  or  in  the 
liquid  form  as  a  lake,  the  heat  expended  by  the  sun  in  lift- 
ing it  has  disappeared  from  the  universe.  It  has  been 
consumed  in  the  act  of  lifting.  But  at  the  moment  that 
the  river  starts  upon  its  downward  course,  and  encounters 
the  resistance  of  its  bed,  the  heat  expended  in  its  eleva- 
tion begins  to  be  restored.  The  mental  eye,  indeed,  can 
follow  the  emission  from  its  source,  through  the  ether  as 
vibratory  motion,  to  the  ocean,  where  it  ceases  to  be  vi- 
bration, and  takes  the  potential  form  among  the  molecules 
of  aqueous  vapour ;  to  the  mountain-top,  where  the  heat 
absorbed  in  vaporization  is  given  out  in  condensation, 
while  that  expended  by  the  sun  in  lifting  the  water  to  its 
present  elevation  is  still  unrestored.  This  we  find  paid 


RELATION   QF   THE   SUN   TO   VEGETABLE   LIFE.         505 

back  to  the  last  unit  by  the  friction  along  the  river's  bed; 
at  the  bottom  of  the  cascades  where  the  plunge  of  the 
torrent  is  suddenly  arrested ;  in  the  warmth  of  the  ma- 
chinery turned  by  the  river ;  in  the  spark  from  the  mill- 
stone; beneath  the  crusher  of  the  miner;  in  the  Alpine 
saw-mill ;  in  the  milk-churn  of  the  chalet ;  in  the  supports 
of  the  cradle  in  which  the  mountaineer,  by  water  power, 
rocks  his  baby  to  sleep.  All  the  forms  of  mechanical  mo- 
tion here  indicated  are  simply  the  parcelling  out  of  an 
amount  of  calorific  motion  derived  originally  from  the 
sun ;  and  at  each  point  at  which  the  mechanical  motion 
is  destroyed,  or  diminished,  it  is  the  sun's  heat  which  is 
restored. 

We  have  thus  far  dealt  with  the  sensible  motions  and 
energies  which  the  sun  produces  and  confers ;  but  there 
are  other  motions  and  other  energies,  whose  relations  are 
not  so  obvious.  Trees  and  vegetables  grow  upon  the 
earth,  and  when  burned  they  give  rise  to  heat,  from  which 
immense  quantities  of  mechanical  energy  are  derived. 
What  is  the  source  of  this  energy  ?  Sir  John  Herschel 
answered  this  question  in  a  general  way ;  while  Dr.  Mayer 
and  Professor  Helmholtz  fixed  its  exact  relation  to  the 
more  general  question  of  conservation.  Let  me  try  to  put 
their  answers  into  plain  words.  You  see  this  iron  rust, 
produced  by  the  falling  together  of  the  atoms  of  iron  and 
oxygen ;  but  though  you  cannot  see  this  transparent  car- 
bonic acid  gas,  it  is  formed  by  the  union  of  carbon  and 
oxygen.  These  atoms  thus  united  resemble  a  weight  rest- 
ing on  the  earth ;  their  mutual  attraction  is  satisfied.  But 
as  I  can  wind  up  the  weight,  and  prepare  it  for  another 
fall,  even  so  these  atoms  can  be  wound  up,  separated  from 
each  other,  and  thus  enabled  to  repeat  the  process  of  com- 
bination. 

In  the  building  of  plants,  carbonic  acid  is  the  material 
from  which  the  carbon  of  the  plant  is  derived,  while  water 
22 


506  LECTURE    XIII. 

is  the  substance  from  which  it  obtains  its  hydrogen.  The 
solar  beam  winds  up  the  weight ;  it  is  the  agent  which 
severs  the  atoms,  setting  the  oxygen  free,  and  allowing 
the  carbon  and  the  hydrogen  to  aggregate  in  woody  fibre. 
If  the  sun's  rays  fall  upon  a  surface  of  sand,  the  sand 
is  heated,  and  finally  radiates  away  as  much  heat  as  it  re- 
ceives ;  but  let  the  same  beams  fall  upon  a  forest ;  then 
the  quantity  of  heat  given  back  is  less  than  that  received, 
for  a  portion  of  the  sunbeams  is  invested  in  the  building 
of  the  trees.  We  have  already  seen  how  heat  is  consumed 
in  forcing  asunder  the  atoms  of  bodies ;  and  how  it  reap- 
pears, when  the  attraction  of  the  separated  atoms  comes 
again  into  play.*  The  precise  considerations  which  we 
then  applied  to  heat,  AVC  have  now  to  apply  to  light,  for  it 
is  at  the  expense  of  the  solar  light  that  the  chemical  de- 
composition takes  place.  Without  the  sun,  the  reduction 
of  the  carbonic  acid  and  water  cannot  be  effected ;  and,  in 
this  act,  an  amount  of  solar  energy  is  consumed,  exactly 
equivalent  to  the  molecular  work  done. 

Combustion  is  the  reversal  of  this  process  of  reduction, 
and  all  the  energy  invested  in  a  plant  reappears  as  heat, 
when  the  plant  is  burned.  I  ignite  this  bit  of  cotton,  it 
bursts  into  flame ;  the  oxygen  again  unites  with  its  car- 
bon, and  an  amount  of  heat  is  given  out,  equal  to  that 
originally  sacrificed  by  the  sun  to  form  the  bit  of  cotton. 
So  also  as  regards  the  *  deposits  of  dynamical  efficiency ' 
laid  up  in  our  coal  strata ;  they  are  simply  the  sun's  rays 
in  a  potential  form.  We  dig  from  our  pits,  annually, 
eighty-four  millions  of  tons  of  coal,  the  mechanical  equiv- 
alent of  which  is  of  almost  fabulous  vastness.  The  com- 
bustion of  a  single  pound  of  coal  in  one  minute  is  equal 
to  the  work  of  three  hundred  horses  for  the  same  time. 
It  would  require  one  hundred  and  eight  millions  of  horses, 

*  Lecture  V. 


RELATION   OF  THE   SUN  TO   ANIMAL   LIFE.  507 

working  day  and  night  with  unimpaired  strength  for  a 
year,  to  perform  an  amount  of  work  equivalent  to  the  en- 
ergy which  the  sun  of  the  Carboniferous  epoch  invested  in 
one  year's  produce  of  our  coalpits. 

The  further  we  pursue  this  subject,  the  more  its  inter- 
est and  its  wonder  grow  upon  us.  I  have  shown  you  how 
a  sun  may  be  produced  by  the  mere  exercise  of  gravita- 
ting force ;  that  by  the  collision  of  cold  dark  planetary- 
masses  the  light  and  heat  of  our  central  orb,  and  also  of 
the  fixed  stars,  may  be  obtained.  But  here  we  find  the 
physical  powers,  derived  or  derivable  from  the  action  of 
gravity  upon  dead  matter,  introducing  themselves  at  the 
very  root  of  the  question  of  vitality.  We  find  in  solar 
light  and  heat  the  very  mainspring  of  vegetable  life. 

Nor  can  we  halt  at  the  vegetable  world,  for  it,  me 
diately  or  immediately,  is  the  source  of  all  animal  life. 
Some  animals  feed:  directly  on  plants,  others  feed  upon 
their  herbivorous  fellow-creatures ;  but  all  in  the  long  run 
derive  life  and  energy  from  the  vegetable  world ;  all,  there- 
fore, as  Helmholtz  has  remarked,  may  trace  their  lineage 
to  the  sun.  In  the  animal  body  the  carbon  and  hydrogen 
of  the  vegetable  are  again  brought  into  contact  with  the 
oxygen  from  which  they  had  been  divorced,  and  which  is 
now  supplied  by  the  lungs.  Reunion  takes  place,  and  ani- 
mal heat  is  the  result.  Save  as  regards  intensity,  there  is 
no  difference  between  the  combustion  that  thus  goes  on 
within  us,  and  that  of  an  ordinary  fire.  The  products  of 
combustion  are  in  both  cases  the  same,  namely,  carbonic 
acid  and  water.  Looking  then  at  the  physics  of  the  ques- 
tion, we  see  that  the  formation  of  a  vegetable  is  a  process 
of  winding  up,  while  the  formation  of  an  animal  is  a  pro- 
cess of  running  down.  This  is  the  rhythm  of  Nature  as 
applied  to  animal  and  vegetable  life. 

But  is  there  nothing  in  the  human  body  to  liberate  it 
from  that  chain  of  necessity  which  the  law  of  conservation 


508  LECTURE   XIII. 

coils  around  inorganic  nature  ?  Look  at  two  men  upon  a 
mountain  side,  with  equal  health  and  physical  strength ; 
the  one  will  sink  and  fail,  while  the  other,  with  determined 
energy,  scales  the  summit.  Has  not  volition,  in  this  case 
a  creative  power?  Physically  considered,  the  law  that 
rules  the  operations  of  a  steam-engine  rules  the  operations 
of  the  climber.  For  every  pound  raised  by  the  former,  an 
equivalent  quantity  of  the  heat  disappears ;  and  for  every 
step  the  climber  ascends,  an  amount  of  heat,  equivalent 
jointly  to  his  own  weight  and  the  height  to  which  it  is 
raised,  is  lost  to  his  body.  The  strong  will  can  draw 
largely  upon  the  physical  energy  furnished  by  the  food  ; 
but  it  can  create  nothing.  The  function  of  the  will  is  to 
apply  and  direct^  not  to  create. 

I  have  just  said,  that  as  a  climber  ascends  a  mountain, 
heat  disappears  from  his  body ;  the  same  statement  applies 
to  animals  performing  work.  It  would  appear  to  follow 
from  this,  that  the  body  ought  to  grow  colder,  in  the  act 
of  climbing  or  of  working,  whereas  universal  experience 
proves  it  to  grow  warmer.  The  solution  of  this  seeming 
contradiction  is  found  in  the  fact,  that  when  the  muscles 
are  exerted,  augmented  respiration  and  increased  chemical 
action  set  in.  The  bellows  which  urge  oxygen  into  the 
fire  within  are  more  briskly  blown,  and  thus,  though  heat 
actually  disappears  as  we  climb,  the  loss  is  more  than  cov- 
jered  by  the  increased  activity  of  the  chemical  processes. 

Heat  is  developed  in  a  muscle  w^hen.  it  contracts,  as 
was  proved  by  MM.  Becquerel  and  Breschet,  by  means  of 
a  modification  of  our  thermo-electric  pile.  MM.  Billroth 
and  Fick  have  found  that  in  the  case  of  persons  who  die 
from  tetanus,  the  temperature  of  the  muscles  is  sometimes 
nearly  eleven  degrees  Fahrenheit  in  excess  of  the  normal 
temperature.  M.  Helmholtz  has  found  that  the  muscles 
of  dead  frogs  in  contracting  produce  heat ;  and  an  ex- 
tremely important  result  as  regards  the  influence  of  con- 


RELATION   OF   THE    SUN   TO   ANIMAL   LIFE.  509 

traction  has  been  obtained  by  Professor  Ludwig  of  Vienna 
and  his  pupils.  Arterial  blood,  you  know,  is  charged  with 
oxygen :  when  this  blood  passes  through  a  muscle  in  an 
ordinary  uncontracted  state,  it  is  changed  into  venous 
blood,  which  still  retains  about  7|-  per  cent,  of  oxygen. 
But  if  the  arterial  blood  pass  through  a  contracted  muscle, 
it  is  almost  wholly  deprived  of  its  oxygen,  the  quantity 
remaining  amounting,  in  some  cases,  to  only  l-f^  per  cent. 
As  a  result  of  the  augmented  combustion  within  the  mus- 
cles when  in  a  state  of  activity,  we  have  an  increased 
amount  of  carbonic  acid  expired  from  the  lungs.  Dr.  Ed- 
ward Smith  has  shown  that  the  quantity  of  this  gas  ex- 
pired during  periods  of  great  exertion  may  be  five  times 
that  expired  in  a  state  of  repose. 

Now  when  we  augment  the  temperature  of  the  body 
by  labour,  a  portion  only  of  the  excess  of  heat  generated 
is  applied  to  the  performance  of  the  work.  Suppose  a 
certain  amount  of  food  to  be  oxidized,  that  is  to  say,  burnt, 
in  the  body  of  a  man  in  a  state  of  repose,  the  quantity  of 
heat  produced  in  the  process  is  exactly  that  which  we 
should  obtain  from  the  direct  combustion  of  the  food  in 
an  ordinary  fire.  But  suppose  the  oxidation  of  the  food 
to  take  place  while  the  man  is  performing  work,  then  the 
heat  generated  in  the  body  falls  short  of  that  which  could 
be  obtained  from  direct  combustion.  An  amount  of  heat 
is  missing,  equivalent  to  the  work  done.  Supposing  the 
work  to  consist  in  the  development  of  heat  by  friction, 
then  the  amount  of  heat  thus  generated  outside  of  the 
man's  body  would  be  exactly  that  which  was  wanting 
within  the  body,  to  make  the  heat  there  generated  equal 
to  that  produced  by  direct  combustion. 

It  is,  of  course,  easy  to  determine  the  amount  of  heat 
consumed  by  a  mountaineer,  in  lifting  his  own  body  to 
any  elevation.  When  lightly  clad,  I  weigh  10  stone,  or 
140  Ibs. ;  what  is  the  amount  of  heat  consumed,  in  my 


510  LECTURE   XHI. 

case,  in  climbing  from  the  sea-level  to  the  top  of  Mont 
Blanc?  The  height  of  the  mountain  is  15, 774  feet;  and 
for  every  pound  of  my  body  raised  to  a  height  of  772  feet, 
a  quantity  of  heat  is  consumed,  sufficient  to  raise  the  tem- 
perature of  a  pound  of  water  1°  Fahr.  Consequently,  on 
climbing  to  a  height  of  15,774,  or  about  20^  times  772 
feet,  I  consume  an  amount  of  heat  sufficient  to  raise  the 
temperature  of  140  Ibs.  of  water  20  J°  Fahr.  If,  on  the 
other  hand,  I  could  seat  myself  at  the  top  of  the  mountain 
and  perform  a  glissade  to  the  sea-level,  the  quantity  of  heat 
generated  by  the  descent  would  be  precisely  equal  to  that 
consumed  in  the  ascent.  I  have  had  occasion  more  than 
once  to  direct  your  attention  to  the  energy  of  molecular 
forces,  and  I  would  do  so  here  once  more.  Measured  by 
one's  feelings,  the  amount  of  exertion  necessary  to  reach 
the  top  of  Mont  Blanc  is  very  great.  Still,  the  energy 
which  performs  this  feat  would  be  derived  from  the  com- 
bustion of  about  two  ounces  of  carbon.  In  the  case  of  an 
excellent  steam-engine,  about  one-tenth  of  the  heat  em- 

C*  " 

ployed  is  converted  into  work ;  the  remaining  nine-tenths 
being  wasted  in  the  air,  the  condenser,  &c.  In  the  case 
of  an  active  mountaineer,  as  much  as  one-fifth  of  the  heat 
due  to  the  oxidation  of  his  food  may  be  converted  Into 
work ;  hence,  as  a  working  machine,  the  animal  body  is 
much  more  perfect  than  the  steam-engine. 

We  see,  however,  that  the  engine  and  the  animal  de- 
rive, or  may  derive  these  powers  from  the  selfsame  source. 
"We  can  work  an  engine  by  the  direct  combustion  of  the 
substances  which  we  employ  as  food ;  and  if  our  stomachs 
were  so  constituted  as  to  digest  coal,  we  should,  as  Ilelrn- 
holtz  has  remarked,*  be  able  to  derive  our  energy  from 
this  substance.  The  grand  point  permanent  throughout 
all  these  considerations  is,  that  nothing  is  created.  Wo 

*  Phil.  Mag.  1856,  vol.  ix.  p.  510. 


EFFECT   OF   THE   SUN   ON   VITAL   ACTIONS.  511 

can  make  no  movement  which  is  not  accounted  for  by  the 
contemporaneous  extinction  of  some  other  movement. 
And  how  complicated  soever  the  motions  of  animals  may 
be,  whatever  may  be  the  change  which  the  molecules  of 
our  food  undergo  within  our  bodies,  the  whole  energy  of 
animal  life  consists  in  the  falling  of  the  atoms  of  carbon 
and  hydrogen  and  nitrogen  from  the  high  level  which 
they  occupy  in  the  food,  to  the  low  level  which  they  oc- 
cupy when  they  quit  the  body.  But  what  has  enabled 
the  carbon  and  the  hydrogen  to  fall  ?  What  first  raised 
them  to  the  level  which  rendered  the  fall  possible  ?  We 
have  already  learned  that  it  is  the  sun.  It  is  at  his  cost 
that  animal  heat  is  produced,  and  animal  motion  accom- 
plished. Not  only  then  is  the  sun  chilled,  that  we  may 
have  our  fires,  but  he  is  likewise  chilled  that  we  may  have 
our  powers  of  locomotion. 

The  subject  is  of  such  vast  importance,  and  is  so  sure 
to  tinge  the  whole  future  course  of  philosophic  thought, 
that  I  will  dwell  upon  it  a  little  longer.  I  will  endeavour, 
by  reference  to  analogical  processes,  to  give  you  a  clearer 
idea  of  the  part  played  by  the  sun  in  vital  actions.  We 
can  raise  water  by  mechanical  action  to  a  high  level ;  and 
that  water,  in  descending  by  its  own  gravity,  may  be 
made  to  assume  a  variety  of  forms,  and  to  perform  various 
kinds  of  mechanical  work.  It  may  be  made  to  fall  in  cas- 
cades, rise  in  fountains,  twirl  in  the  most  complicated  ed- 
dies, or  flow  along  a  uniform  bed.  It  may,  moreover,  be 
employed  to  turn  wheels,  wield  hammers,  grind  corn,  or 
drive  piles.  Now  there  is  no  power  created  by  the  water 
during  its  descent.  All  the  energy  which  it  exhibits  is 
merely  the  parcelling  out  and  distribution  of  the  oiiginal 
energy  which  raised  it  up  on  high.  Thus  also  as  regards 
the  complex  motions  of  a  clock  or  a  watch ;  they  are  en- 
tirely derived  from  the  energy  of  the  hand  which  winds 
it  up.  Thus  also  the  singing  of  the  little  Swiss  bird  in 


512  LECTURE   XIII. 

the  International  Exhibition  of  1862;  the  quivering  of  its 
artificial  organs,  the  vibrations  of  the  air  which  strike  the 
ear  as  melody,  the  flutter  of  its  little  wings,  and  all  other 
motions  of  the  pretty  automaton,  were  simply  derived 
from  the  force  by  which  it  was  wound-up.  It  gives  out 
nothing  that  it  has  not  received.  In  this  precise  sense, 
you  will  perceive,  is  the  energy  of  man  and  animals,  the 
parcelling  out  and  distribution  of  an  energy  originally 
exerted  by  the  sun.  In  the  vegetable,  as  we  have  re- 
marked, the  act  of  elevation,  or  of  winding-up,  is  per- 
formed ;  and  it  is  during  the  descent,  in  the  animal,  of  the 
carbon,  hydrogen,  and  nitrogen,  to  the  level  from  which 
they  started,  that  the  powers  of  life  appear. 

But  the  question  is  not  yet  exhausted.  The  water 
which  we  used  in  our  first  illustration  produces  all  the 
motion  displayed  in  its  descent,  but  the  form  of  the  motion 
depends  on  the  character  of  the  machinery  interposed  in 
the  path  of  the  water.  And  thus  the  primary  action  of 
the  sun's  rays  is  qualified  by  the  atoms  and  molecules 
among  which  their  power  is  distributed.  Molecular  forces 
determine  the  form  which  the  solar  energy  will  assume. 
In  the  one  case  this  energy  is  so  conditioned  by  its 
atomic  machinery  as  to  result  in  the  formation  of  a  cab- 
bage ;  in  another  case  it  is  so  conditioned  as  to  result  in 
the  formation  of  an  oak.  So  also  as  regards  the  reunion 
of  the  carbon  and  the  oxygen — the  form  of  their  reunion 
is  determined  by  the  molecular  machinery  through  which 
the  combining  force  acts.  In  one  case  the  action  may  re- 
sult in  the  formation  of  a  man,  while  in  another  it  may 
result  in  the  formation  of  a  grasshopper. 

The  matter  of  our  bodies  is  that  of  inorganic  nature. 
There  is  no  substance  in  the  animal  tissues  which  is  not 
primarily  derived  from  the  rocks,  the  water,  and  the  air. 
Are  the  forces  of  organic  matter,  then,  different  in  kind 
from  those  of  inorganic  ?  All  the  philosophy  of  the  pres- 


CONCLUSION.  513 

ent  day  tends  to  negative  the  question ;  and  to  show  that 
it  is  the  directing  and  compounding,  in  the  inorganic  world, 
of  forces  belonging  equally  to  the  inorganic,  that  consti- 
tutes the  mystery  and  the  miracle  of  vitality. 

In  discussing  the  material  combinations  which  result 
in  the  formation  of  the  body  and  the  brain  of  man,  it  is 
impossible  to  avoid  taking  side  glances  at  the  phenomena 
of  consciousness  and  thought.  Science  has  asked  daring 
questions,  and  will,  no  doubt,  continue  to  ask  such.  Prob- 
lems will  assuredly  present  themselves  to  men  of  a  future 
age,  which,  if  enunciated  now,  would  appear  to  most 
people  as  the  direct  offspring  of  insanity.  Still,  though 
the  progress  and  development  of  science  may  seem  to  be 
unlimited,  there  is  a  region  apparently  beyond  her  reach 
— a  line,  with  which  she  does  not  even  tend  to  osculate. 
Given  the  masses  and  distances  of  the  planets,  we  can  in- 
fer the  perturbations  consequent  on  their  mutual  attrac- 
tions. Given  the  nature  of  a  disturbance  in  water,  air,  or 
ether,  we  can  infer  from  the  properties  of  the  medium  how 
its  particles  will  be  affected.  In  all  this  we  deal  with 
physical  laws,  and  the  mind  runs  along  the  line  which 
connects  the  phenomena  from  beginning  to  end.  But 
when  we  endeavour  to  pass,  by  a  similar  process,  from  the 
region  of  physics  to  that  of  thought,  we  meet  a  problem 
to  seize  on  which  transcends  any  conceivable  expansion 
of  the  powers  we  now  possess.  We  may  think  over  the 
subject  again  and  again,  but  it  eludes  all  intellectual  pre- 
sentation. Thus,  though  the  territory  of  science  is  wide, 
it  has  its  limits,  from  which  we  look  with  vacant  gaze 
into  the  region  beyond.  We  may  fairly  claim  matter  in 
all  its  forms,  not  only  as  it  appears  in  external  nature ; 
but  even  as  it  exists  in  the  muscles,  blood,  and  brain  of 
man  himself,  it  is  ours  to  experiment  and  speculate  upon. 
Rejecting  the  idea  of  a  '  vital  force,'  let  us  reduce,  if  we 
can,  the  physical  phenomena  of  life  to  attractions  and 
22* 


514  LECTURE  xin. 

repulsions.  But  having  thus  exhausted  physics,  and 
reached  its  very  rim,  the  real  mystery  yet  looms  beyond 
us.  And  thus  it  will  ever  loom — ever  beyond  the  bourne 
of  man's  intellect — giving  the  poets  of  successive  ages 
just  occasion  to  declare  that 

We  are  such  stuff 

As  dreams  are  made  of,  and  our  little  life 
Is  rounded  by  a  sleep. 

Still,  presented  rightly  to  the  mind,  the  discoveries 
and  generalisations  of  modern  science  constitute  a  poem 
more  sublime  than  has  ever  yet  been  addressed  to  the 
imagination.  The  natural  philosopher  of  to-day  may  dwell 
amid  conceptions  which  beggar  those  of  Milton.  So  great 
and  grand  are  they,  that  in  the  contemplation  of  them 
a  certain  force  of  character  is  requisite  to  preserve  us 
from  bewilderment.  Look  at  the  integrated  energies  of 
our  world, — the  stored  power  of  our  coal-fields;  our  winds 
and  rivers;  our  fleets,  armies,  and  guns.  What  are  they? 
They  are  all  generated  by  a  portion  of  the  sun's  energy, 
which  does  not  amount  to  -g-gTroo^ooinr  of  the  whole.  This 
is  the  entire  fraction  of  the  sun's  force  intercepted  by  the 
earth,  and  we  convert  but  a  small  fraction  of  this  fraction 
into  mechanical  energy.  Multiplying  all  our  powers  by 
millions  of  millions,  we  do  not  reach  the  sun's  expenditure. 
And  still,  notwithstanding  this  enormous  drain,  in  the 
lapse  of  human  history  we  are  unable  to  detect  a  diminu- 
tion of  his  store.  Measured  by  our  largest  terrestrial 
standards,  such  a  reservoir  of  power  is  infinite ;  but  it  is 
our  privilege  to  rise  above  these  standards,  and  to  regard 
the  sun  himself  as  a  speck  in  infinite  extension — a  mere 
drop  in  the  universal  sea.  We  analyse  the  space  in  which 
he  is  immersed,  and  which  is  the  vehicle  of  his  power. 
We  pass  to  other  systems  and  other  suns,  each  pouring 
forth  energy  like  our  own,  but  still  without  infringement 


CONCLUSION.  515 

of  the  law,  which  reveals  immutability  in  the  midst  of 
change,  which  recognizes  incessant  transference  or  conver- 
sion, but  neither  final  gain  nor  loss.  This  law  generalises 
the  aphorism  of  Solomon,  that  there  is  nothing  new  under 
the  sun,  by  teaching  us  to  detect  everywhere,  under  its 
infinite  variety  of  appearances  the  same  primeval  force. 
To  Nature  nothing  can  be  added ;  from  Nature  nothing 
can  be  taken  away ;  the  sum  of  her  energies  is  constant, 
and  the  utmost  man  can  do  in  the  pursuit  of  physical 
truth,  or  in  the  applications  of  physical  knowledge,  is  to 
shift  the  constituents  of  the  never-varying  total.  The  law 
of  conservation  rigidly  excludes  both  creation  and  annihi- 
lation. Waves  may  change  to  ripples,  and  ripples  to 
waves — magnitude  may  be  substituted  for  number,  and 
number  for  magnitude — asteroids  may  aggregate  to  suns, 
suns  may  resolve  themselves  into  florae  and  fauna?,  and 
flora)  and  faunas  melt  in  air — the  flux  of  power  is  eternally 
the  same — it  rolls  in  music  through  the  ages,  and  all  ter- 
restrial energy — the  manifestations  of  life  as  well  as  the 
display  of  phenomena — are  but  the  modulations  of  its 
rhythm. 


APPENDIX  TO  LECTURE  XIII. 


EXTRACT  FROM  A  LECTURE  '  ON  THE  PHYSICAL  BASIS  OF 
SOLAR  CHEMISTRY.'* 

WE  have  now  some  hard  work  before  us ;  hitherto  we  have  been 
delighted  by  objects  which  addressed  themselves  rather  to  our 
aesthetic  taste  than  to  our  scientific  faculty.  "We  have  ridden 
pleasantly  to  the  base  of  the  final  cone  of  Etna,  and  must  now  dis- 
mount and  march  wearily  through  ashes  and  lava,  if  we  would 
enjoy  the  prospect  from  the  summit.  Our  problem  is  to  connect 
the  dark  lines  of  Fraunhofer  with  the  bright  ones  of  the  metals. 
The  white  beam  of  the  lamp  is  refracted  in  passing  through  our 
two  prisms,  but  its  different  components  are  refracted  in  different 
degrees,  and  thus  its  colours  are  drawn  apart.  Now  the  colour 
depends  solely  upon  the  rate  of  oscillation  of  the  particles  of  the 
luminous  body ;  red  light  being  produced  by  one  rate,  blue  light 
by  a  much  quicker  rate,  and  the  colours  between  red  and  blue  by 
the  intermediate  rates.  The  solid  incandescent  coal-points  give  us 
a  continuous  spectrum ;  or,  in  other  words,  they  emit  rays  of  all 
possible  periods  between  the  two  extremes  of  the  spectrum. 
They  have  particles  oscillating  so  as  to  produce  red ;  others,  to 
produce  orange;  others,  to  produce  yellow,  green,  blue,  indigo, 
and  violet  respectively.  Colour,  as  many  of  you  know,  is  to  light 
what  pitch  is  to  sound.  When  a  violin-player  presses  his  finger 
on  a  string  he  makes  it  shorter  and  tighter,  and  thus,  causing  it  to 
vibrate  more  speedily,  augments  the  pitch.  Imagine  such  a  player 
to  move  his  finger  slowly  along  the  string,  shortening  it  gradually 
as  he  draws  his  bow,  the  note  would  rise  in  pitch  by  a  regular 

*  Given  at  the  Royal  Institution  on  Friday  evening,  June  7,  1861. 


PHYSICAL   BASIS    OF    SOLAE    CHEMISTRY.  517 

gradation ;  there  would  be  no  gap  intervening  between  note  and 
note.  Here  we  have  the  analogue  to  the  continuous  spectrum, 
whose  colours  insensibly  blend  together  without  gap  or  interrup- 
tion, from  the  red  of  the  lowest  pitch  to  the  violet  of  the  highest. 
But  suppose  the  player,  instead  of  gradually  shortening  his  string, 
to  press  his  finger  on  a  certain  point,  and  to  sound  the  correspond- 
ing note ;  then  to  pass  on  to  another  point  more  or  less  distant, 
and  sound  its  note ;  then  to  another,  and  so  on,  thus  sounding  par- 
ticular notes  separated  from  each  other  by  gaps  which  correspond 
to  the  intervals  of  the  string  passed  over ;  we  should  then  have 
the  exact  analogue  of  a  spectrum  composed  of  separate  bright 
bands  with  intervals  of  darkness  between  them.  But  this,  though 
a  perfectly  true  and  intelligible  analogy,  is  not  sufficient  for  our 
purpose;  we  must  look  with  the  mind's  eye  at  the  very  oscillating 
atoms  of  the  volatilised  metal.  Figure  these  atoms  connected  by 
springs  of  a  certain  tension,  which,  when  the  atoms  are  squeezed 
together,  push  them  asunder,  and  when  the  atoms  are  drawn  apart, 
pull  them  together,  causing  them,  before  coming  to  rest,  to  quiver/ 
at  a  certain  definite  rate  determined  by  the  strength  of  the  spring. 
Now  the  volatilised  metal  which  gives  us  one  bright  band  is  to  be 
figured  as  having  its  atoms  united  by  springs  all  of  the  same  ten- 
sion, its  vibrations  are  all  of  one  kind.  The  metal  which  gives  us 
two  bands  may  be  figured  as  having  some  of  its  atoms  united  by 
springs  of  one  tension,  and  others  by  a  second  series  of  springs  of 
a  different  tension.  Its  vibrations  are  of  two  distinct  kinds ;  so 
also  when  we  have  three  or  more  bands,  we  are  to  figure  as  many 
distinct  sets  of  springs,  each  set  capable  of  vibrating  in  its  own 
particular  time  and  at  a  different  rate  from  the  others.  If  we 
seize  this  idea  definitely,  we  shall  have  no  difficulty  in  dropping 
the  metaphor  of  springs,  and  substituting  for  it  mentally  the  forces 
by  which  the  atoms  act  upon  each  other.  Having  thus  far  cleared 
our  way,  let  us  make  another  effort  to  advance. 

Here  is  a  pendulum — a  heavy  ivory  ball  suspended  from  a 
string.  I  blow  against  this  ball ;  a  single  puff  of  my  breath  moves 
it  a  little  way  from  its  position  of  rest ;  it  swings  back  towards 
me,  and  when  it  reaches  the  limit  of  its  swing  I  puff  again.  It 
now  swings  farther ;  and  thus  by  timing  my  puffs  I  can  so  accumu- 
late their  action  as  to  produce  oscillations  of  large  amplitude.  The 
ivory  ball  here  has  absorbed  the  motion  which  my  breath  coinrnu- 


518  APPENDIX   TO    LECTURE   XIII. 

nicated  to  the  air.  I  now  bring  the  ball  to  rest.  Suppose,  instead 
of  my  breath,  a  wave  of  air  strike  against  it,  and  that  this  wave 
is  followed  by  a  series  of  others  which  succeed  each  other  in  the 
same  intervals  as  my  puffs;  it  is  perfectly  manifest  that  these 
waves  would  communicate  their  motion  to  the  ball  and  cause  it  to 
swing  as  the  puffs  did.  And  it  is  equally  manifest  that  this  would 
not  be  the  case  if  the  impulses  of  the  waves  were  not  properly 
timed ;  for  then  the  motion  imparted  to  the  pendulum  by  one  wave 
would  be  neutralized  by  another,  and  there  could  not  be  that  accu- 
mulation of  effect  which  we  have  when  the  periods  of  the  waves 
correspond  with  the  periods  of  the  pendulum.  So  much  for  the 
kind  of  impulses  absorbed  by  the  pendulum.  But  such  a  pendu- 
lum set  oscillating  in  air  produces  waves  in  the  air ;  and  we  see 
that  the  waves  which  it  produces  must  be  of  the  same  period  as 
those  whose  motions  it  would  take  up  or  absorb  most  copiously  if 
they  struck  against  it.  Just  in  passing  I  may  remark,  that  if  the 
periods  of  the  waves  be  double,  treble,  quadruple,  &c.,  the  periods 
of  the  pendulum,  the  shocks  imparted  to  the  latter  would  also  be 
so  timed  as  to  produce  an  accumulation  of  motion. 

Perhaps  the  most  curious  effect  of  these  timed  impulses  ever 
described,  was  that  observed  by  a  watchmaker,  named  EUicott,  in 
the  year  1741.  He  set  two  clocks  "leaning  against  the  same  rail: 
one  of  them,  which  we  may  call  A,  was  set  going ;  the  other,  B, 
not.  Some  time  afterwards  he  found,  to  his  surprise,  that  B  was 
ticking  also.  The  pendulums  being  of  the  same  length,  the  shocks 
imparted  by  the  ticking  of  A  to  the  rail  against  which  both  clocks 
rested,  were  propagated  to  B,  and  were  so  timed  as  to  set  B  going. 
Other  curious  effects  were  at  the  same  time  observed.  When  the 
pendulums  differed  from  each  other  a  certain  amount,  A  set  B 
going.  But  the  reaction  of  B  stopped  A.  Then  B  set  A  going, 
and  the  reaction  of  A  stopped  B.  If  the  periods  of  oscillation 
were  close  to  each  other, 'but  still  not  quite  alike,  the  clocks  con- 
trolled each  other,  and  by  a  kind  of  mutual  compromise  they  ticked 
in  perfect  unison. 

But  what  has  all  this  to  do  with  our  present  subject?  The 
questions  are  mechanically  identical,  the  varied  actions  of  the  uni- 
verse are  all  modes  of  motion ;  and  the  vibration  of  a  ray  claims 
strict  brotherhood  with  the  vibrations  of  our  pendulum.  Suppose 
ethereal  waves  striking  upon  atoms  which  oscillate  in  periods  the 


PHYSICAL   BASIS   OF   SOLAR   CHEMISTKY.  519 

same  as  those  in  which  the  waves  succeed  each  other,  the  motion 
of  the  waves  will  be  absorbed  by  the  atoms  ;  suppose  we  send  our 
beam  of  white  light  through  a  sodium  flame,  the  particles  of  that 
flame  will  be  chiefly  affected  by  those  undulations  which  are  syn- 
chronous with  their  own  periods  of  vibration.  There  will  be  on 
the  part  of  those  particular  rays  a  transference  of  motion  from  the 
agitated  ether  to  the  atoms  of  the  volatilised  sodium,  which,  as 
already  defined,  is  absorption.  We  use  glass  screens  to  defend  us 
from  the  heat  of  our  fires:  how  do  they  act?  Thus: — The  heat 
emanating  from  the  fire  is  for  the  most  part  due  to  rays  which  are 
incompetent  to  excite  the  sense  of  vision ;  we  call  these  rays  ob- 
scure. Glass,  though  pervious  to  the  luminous  rays,  is  opaque  in 
a  high  degree  to  those  obscure  rays,  and  cuts  them  off,  while  the 
cheerful  light  of  the  fire  is  allowed  to  pass.  Now  mark  me  clearly. 
The  heat  cut  off  from  your  person  is  to  be  found  in  the  glass,  the 
latter  becomes  heated  and  radiates  towards  your  person;  what 
then  is  the  use  of  the  glass  if  it  merely  thus  acts  as  a  temporary 
halting-place  for  the  rays,  and  sends  them  on  afterwards?  It  does 
this : — It  not  only  sends  the  heat  it  receives  towards  you,  but  scat- 
ters it  also  in  all  other  directions,  round  the  room.  Thus  the  rays 
which,  were  the  glass  not  interposed,  would  be  shot  directly  against 
your  person,  are  for  the  most  part  diverted  from  their  original  di- 
rection, and  you  are  preserved  from  their  impact. 

Now  for  our  experiment.  I  pass  the  beam  from  the  electric 
lamp  through  the  two  prisms,  and  the  spectrum  spreads  its  colours 
upon  the  screen.  Between  the  lamp  and  the  prism  I  interpose 
this  snapdragon  light.  Alcohol  and  water  are  here  mixed  up  with 
a  quantity  of  common  salt,  and  the  metal  dish  that  contains  them 
is  heated  by  a  spirit  lamp.  The  vapour  from  the  mixture  ignites, 
and  we  have  this  monochromatic  flame.  Through  this  flame  the 
beam  from  the  lamp  is  now  passing,  and  observe  the  result  upon 
the  spectrum.  You  see  a  dark  band  cut  out  of  the  yellow — not 
very  dark,  but  sufficiently  so  to  be  seen  by  everybody  present. 
Observe  how  the  band  quivers  and  varies  in  shade,  as  the  yellow 
light  cut  off  by  the  unsteady  flame  varies  in  amount.  The  flame 
of  this  monochromatic  lamp  is  at  the  present  moment  casting  its 
proper  yellow  light  upon  that  shaded  line ;  and  more  than  this,  it 
casts,  in  part,  the  light  which  it  absorbs  from  the  electric  lamp 
upon  it ;  but  it  scatters  the  greater  portion  of  this  light  in  other 


520  APPENDIX   TO   LECTURE   XIII. 

directions,  and  thus  withdraws  it  from  its  place  upon  the  screen, 
as  the  glass,  in  the  case  above  supposed,  diverted  the  heat  of  the 
lire  from  your  person.  Hence  the  band  appears  dark;  not  abso- 
lutely, but  dark  in  comparison  with  the  adjacent  brilliant  portions 
of  the  spectrum. 

But  let  me  exalt  this  effect.  I  place  in  front  of  the  electric 
lamp  the  intense  flame  of  a  large  Bunsen's  burner.  I  have  here  a 
platinum  spoon  in  which  I  put  a  bit  of  sodium  less  than  a  pea  in 
magnitude.  The  sodium  placed  in  the  flame  soon  volatilises  and 
burns  with  brilliant  incandescence.  Observe  the  spectrum.  The 
yellow  band  is  clearly  and  sharply  cut  out,  and  a  band  of  intense 
obscurity  occupies  its  place.  I  withdraw  the  sodium,  the  brilliant 
yellow  of  the  spectrum  takes  its  proper  place :  I  reintroduce  the 
sodium,  and  the  black  band  appears. 

Let  me  be  more  precise : — The  yellow  colour  of  the  spectrum 
extends  over  a  sensible  space,  blending  on  one  side  into  orange 
and  on  the  other  into  green.  The  term  '  yellow  band '  is  therefore 
somewhat  indefinite.  I  want  to  show  you  that  it  is  the  precise 
yellow  band  emitted  by  the  volatilised  sodium  which,  the  same 
substance  absorbs.  By  dipping  the  coal-point  used  for  the  positive 
electrode  into  a  solution  of  common  salt,  and  replacing  it  in  the 
lamp,  I  obtain  that  bright  yellow  band  which  you  now  see  drawn 
across  the  spectrum.  Observe  the  fate  of  that  band  when  I  inter- 
pose my  sodium  light.  It  is  first  obliterated,  and  instantly  that 
black  streak  occupies  its  place.  See  how  it  alternately  flashes  and 
vanishes  as  I  withdraw  and  introduce  the  sodium  flame. 

And  supposing  that,  instead  of  the  flame  of  sodium  alone,  I 
introduce  into  the  path  of  the  beam  a  flame  in  which  lithium, 
strontium,  magnesium,  calcium,  &c.,  are  in  a  state  of  volatilisation, 
each  metallic  vapour  would  cut  out  its  own  system  of  bands,  each 
corresponding  exactly  in  position  with  the  bright  band  which  that 
metal  itself  would  cast  upon  the  screen.  The  light  of  our  electric 
lamp  then  shining  through  such  a  composite  flame  would  give  us  a 
spectrum  cut  up  by  dark  lines,  exactly  as  the  solar  spectrum  is  cut 
up  by  the  lines  of  Fraunhofer. 

And  hence  we  infer  the  constitution  of  the  great  centre  of  our 
system.  The  sun  consists  of  a  nucleus  which  is  surrounded  by  a 
flaming  atmosphere.  The  light  of  the  nucleus  would  give  us  a 
continuous  spectrum,  as  our  common  coal-points  did ;  but  having 


fr 


PHYSICAL   BASIS   OF   SOLA'S   CHEMISTEY.  521 

to  pass  through  the  photosphere,  as  our  beam  through  the  flame, 
those  rajs  of  the  nucleus  which  the  photosphere  can  itself  emit, 
are  absorbed,  and  shaded  spaces,  corresponding  to  the  particular 
rays  absorbed,  occur  in  the  spectrum.  Abolish  the  solar  nucleus, 
and  we  should  have  a  spectrum  showing  a  bright  band  in  the  place 
of  every  dark  line  of  Fraunhofer.  These  lines  are  therefore  not 
absolutely  dark,  but  dark  by  an  amount  corresponding  to  the  dif- 
ference between  the  light  of  the  nucleus  intercepted  by  the  photo- 
sphere, and  the  light  which  issues  from  the  latter. 

The  man  to  whom  we  owe  this  beautiful  generalisation  is 
Kirchhoff,  Professor  of  Natural  Philosophy  in  the  University  of 
Heidelberg ;  but,  like  every  other  great  discovery,  it  is  compounded 
of  various  elements.  Mr.  Talbot  observed  the  bright  lines  in  the 
spectra  of  coloured  flames.  Sixteen  years  ago  Dr.  Miller  gavo 
drawings  and  descriptions  of  the  spectra  of  various  coloured 
flames.  Wheatstone,  with  his  accustomed  ingenuity,  analysed  the 
light  of  the  electric  spark,  and  showed  that  the  metals  between 
which  the  spark  passed  determined  the  bright  bands  in  the  spec- 
trum of  the  spark.  Masson  published  a  prize  essay  on  these  bands. 
Yan  der  "Willigen,  and  more  recently  Pliicker,  have  given  us  beau- 
tiful drawings  of  the  spectra  obtained  from  the  discharge  of  Kuhm- 
korff's  coil.  But  none  of  these  distinguished  men  betrayed  the 
least  knowledge  of  the  connection  between  the  bright  bands  of  the 
metals  and  the  dark  lines  of  the  solar  spectrum.  The  man  who 
came  nearest  to  the  philosophy  of  the  subject,  was  Angstrom.  In 
a  paper  translated  from  Poggendorff's  '  Annalen '  by  myself,  and 
published  in  the  '  Philosophical  Magazine '  for  1855,  he  indicates 
that  the  rays  which  a  body  absorbs  are  precisely  those  which  it  can 
emit  when  rendered  luminous.  In  another  place,  he  speaks  of  one 
of  his  spectra  giving  the  general  impression  of  reversal  of  the  solar 
spectrum.  Foucault,  Stokes,  Thomson,  and  Stewart,  have  all  been 
very  close  to  the  discovery ;  and,  for  my  own  part,  the  examination 
of  the  radiation  and  absorption  of  heat  by  gases  and  vapours, 
some  of  the  results  of 'which  I  placed  before  you  at  the  commence- 
ment of  this  discourse,  would  have  led  me  in  1859  to  the  law  on 
which  all  Kirchhoff 's  speculations  are  founded,  had  not  an  acci- 
dent withdrawn  me  from  the  investigation.  But  Kirchhoff 's 
claims  are  unaffected  by  these  circumstances.  True,  much  that  I 
have  referred  to  formed  the  necessary  basis  of  his  discovery ;  so 


522  APPENDIX   TO   LECTUKE   XIII. 

did  the  laws  of  Kepler  furnish  to  Newton  the  basis  of  the  theory 
of  gravitation.  But  what  Kirchhoff  has  done  carries  us  far  be- 
yond all  that  had  before  been  accomplished.  He  has  introduced 
the  order  of  law  amid  a  vast  assemblage  of  empirical  observations, 
and  has  ennobled  our  previous  knowledge  by  showing  its  relation- 
ship to  some  of  the  most  sublime  of  natural  phenomena. 


EXTRACT  FROM  A  PAPER  BY  MR.  JOULE. 

In  a  postscript  to  a  paper  in  the  December  number  of  the  '  Phil- 
osophical Magazine '  for  1843,  Mr.  Joule  made  the  following  ex- 
tremely important  remark : — 

'  On  conversing  a  few  days  ago  with  my  friend  Mr.  John  Davies, 
he  told  me  that  he  had  himself  a  few  years  ago  attempted  to  ac- 
count for  that  part  of  animal  heat  which  Crawford's  theory  has 
left  unexplained,  by  the  friction  of  the  blood  in  the  veins  and  arte- 
ries, but  that,  finding  a  similar  hypothesis  in  Haller's  "  Physiol- 
ogy," he  had  not  pursued  the  subject  farther.  It  is  unquestion- 
able that  heat  is  produced  by  such  friction,  but  it  must  be  under- 
stood that  the  mechanical  force  expended  in  the  friction  is  a  part 
of  the  force  of  affinity,  which  causes  the  venous  blood  to  unite 
with  the  oxygen,  so  that  the  whole  heat  of  the  system  must  still 
be  referred  to  the  chemical  changes.  But  if  the  animal  were  en- 
gaged in  turning  a  piece  of  machinery,  or  in  ascending  a  moun- 
tain, I  apprehend  that,  in  proportion  to  the  muscular  effort  put 
forth  for  the  purpose,  a  diminution  of  the  heat  evolved  in  the  sys- 
tem by  a  given  chemical  action  would  be  experienced.' 


EXTRACTS  FROM  DR.  MAYER'S  PAPER  ON  ORGANIC  MOTION 
AND  NUTRITION. 

The  following  brief  extracts  are  from  an  Essay  by  Dr.  Mayer 
on  Organic  Motion  and  Nutrition — one  of  the  most  important  of 
contributions  to  the  science  of  our  time : — 

'Measured  by  human  standards,  the  sun  is  an  inexhaustible 


MAYER   ON   VITAL   DYNAMICS.  523 

source  of  physical  energy.  This  is  the  continually  wound-up 
spring  which  is  the  source  of  nil  terrestrial  activity.  The  vast 
amount  of  force  sent  by  the  earth  into  space  in  the  form  of  wave 
motion  would  soon  bring  its  surface  to  the  temperature  of  death. 
But  the  light  of  the  sun  is  an  incessant  compensation.  It  is  the 
sun's  light,  converted  into  heat,  which  sets  our  atmosphere  in  mo- 
tion, which  raises  the  water  into  clouds,  and  thus  causes  the  rivers 
to  flow.  The  heat  developed  by  friction  in  the  wheels  of  our  wind 
and  water  mills  was  sent  from  the  sun  to  the  earth  in  the  form  of 
vibratory  motion. 

'  Nature  has  proposed  to  herself  the  task  of  storing  up  the  light 
which  streams  earthward  from  the  sun — of  converting  the  most 
volatile  of  all  powers  into  a  rigid  form,  and  thus  preserving  it  for 
her  purposes.  To  this  end  she  has  overspread  the  earth  with  or- 
ganisms, which,  living,  take  into  them  the  solar  light,  and  by  the 
consumption  of  its  energy  generate  incessantly  chemical  forces. 

'  These  organisms  are  plants.  The  vegetable  world  constitutes 
the  reservoir  in  which  the  fugitive  solar  rays  are  fixed,  suitably 
deposited,  and  rendered  ready  for  iiseful  application.  "With  this 
process  the  existence  of  the  human  race  is  inseparably  connected. 
The  reducing  action  of  the  sun's  rays  on  inorganic  and  organic 
substances  is  well  known ;  this  reduction  takes  place  most  copiously 
in  full  sunlight,  less  copiously  in  the  shade,  and  is  entirely  absent 
in  darkness,  and  even  in  candle-light.  The  reduction  is  a  conver- 
sion of  one  form  of  force  into  another — of  mechanical  effect  into 
chemical  tension. 

'  The  time  does  not  lie  far  behind  us  when  it  was  a  subject  of 
contention  whether,  during  life,  plants  did  not  possess  the  power 
of  changing  the  chemical  elements,  and  indeed  of  creating  them. 
Facts  and  experiments  seemed  to  favour  the  notion,  but  a  more 
accurate  examination  has  proved  the  contrary.  We  now  know 
that  the  sum  of  the  materials  employed  and  excreted  is  equal  to 
the  total  quantity  of  matter  taken  up  by  the  plant.  The  tree,  for 
example,  which  weighs  several  thousand  pounds,  has  taken  every 
grain  of  its  substance  from  its  neighbourhood.  In  plants  a  conver- 
sion only,  and  not  a  generation  of  matter,  takes  place. 

*  Plants  consume  the  force  of  light,  and  produce  in  its  place 
chemical  tensions.  Since  the  time  of  Saussure,  the  action  of  light 
has  been  known  to  be  necessary  to  the  reduction^  In  the  first 


524  APPENDIX  TO   LECTURE   XIII. 

place  we  must  enquire  whether  the  light  which  falls  upon  living 
plants  finds  a  different  application  from  that  which  falls  upon  dead 
matter;  that  is  to  say,  whether,  cceteris  paribus,  plants  are  less 
warmed  by  solar  light  than  other  bodies  equally  dark-coloured. 
The  results  of  the  observations  hitherto  made  on  a  small  scale 
seem  to  lie  within  the  limits  of  possible  error.  On  tbe  other  hand, 
every-day  experience  teaches  us  that  the  heating  action  of  the 
sun's  rays  on  large  areas  of  land  is  moderated  by  nothing  more 
powerfully  than  by  a  rich  vegetation,  although  plants,  on  account 
of  the  darkness  of  their  leaves,  must  be  able  to  absorb  a  greater 
quantity  of  heat  than  the  bare  earth.  If,  to  account  for  this  cool- 
ing action,  the  evaporation  from  the  plants  be  not  sufficient,  then 
the  question  above  proposed  must  be  answered  in  the  affirmative. 

'  The  second  question  refers  to  the  cause  of  the  chemical  tension 
produced  in  the  plant.  This  tension  is  a  physical  force.  It  is 
equivalent  to  the  heat  obtained  from  the  combustion  of  the  plant. 
Does  this  force,  then,  come  from  the  vital  processes,  and  without 
the  expenditure  of  some  other  form  of  force?  The  creation  of  a 
physical  force,  of  itself  hardly  thinkable,  seems  all  the  more  para- 
doxical when  we  consider  that  it  is  only  by  the  help  of  the  sun's 
rays  that  plants  can  perform  their  work.  By  the  assumption  of 
such  a  hypothetical  action  of  the  "  vital  force  "  all  further  investi- 
gation is  cut  off,  and  the  application  of  the  methods  of  exact  sci- 
ence to  the  phenomena  of  vitality  is  rendered  impossible.  Those 
who  hold  a  notion  so  opposed  to  the  spirit  of  science  would  be 
thereby  carried  into  the  chaos  of  unbridled  phantasy.  I  therefore 
hope  that  I  may  reckon  on  the  reader's  assent  when  I  state,  as  an 
axiomatic  truth,  that  during  vital  processes  a  conversion  only  of 
matter,  as  well  as  of  force,  occurs,  and  that  a  creation  of  either  the 
one  or  the  other  never  takes  place. 

'  The  physical  force  collected  by  plants  becomes  the  property  of 
another  class  of  creatures — of  animals.  The  living  animal  con- 
sumes combustible  substances  belonging  to  the  vegetable  world, 
and  causes  them  to  reunite  with  the  oxygen  of  the  atmosphere. 
Parallel  to  this  process  runs  the  work  done  by  animals.  This  work 
is  the  end  and  aim  of  animal  existence.  Plants  certainly  produce 
mechanical  effects,  but  it  is  evident  that  for  equal  masses  and  times 
the  sum  of  the  effects  produced  by  a  plant  is  vanishingly  small, 


HAYEK   ON    VITAL   DYNAMICS.  525 

compared  with  those  produced  by  an  animal.  While,  then,  in  the 
plant  the  production  of  mechanical  effects  plays  quite  a  subordinate 
part,  the  conversion  of  chemical  tensions  into  useful  mechanical 
effect  is  the  characteristic  sign  of  fjiimal  life. 

'  In  the  animal  body  chemical  forces  are  perpetually  expended. 
Ternary  and  quaternary  compounds  undergo  during  the  life  of  the 
animal  the  most  important  changes,  and  are,  for  the  most  part, 
given  off  in  the  form  of  binary  compounds — as  burnt  substances. 
The  magnitude  of  these  forces,  with  reference  to  the  heat  devel- 
oped in  these  processes,  is  by  no  means  determined  with  sufficient 
accuracy ;  but  here,  where  our  object  is  simply  the  establishment 
of  a  principle,  it  will  be  sufficient  to  take  into  account  the  heat  of 
combustion  of  the  pure  carbon.  When  additional  data  have  been 
obtained,  it  will  be  easy  to  modify  our  numerical  calculations  so  as 
to  render  them  accordant  with  the  new  facts. 

'  The  heat  of  combustion  of  carbon  I  assume  with  Dulong  to  be 
8550°.*  The  mechanical  work  which  corresponds  to  the  combus- 
tion of  one  unit  of  weight  of  coal  corresponds  to  the  raising  of 
9,670,000  units  to  a  height  of  1  foot. 

'  If  we  express  by  a  weight  of  carbon  the  quantity  of  chemical 
force  wrhich  a  horse  must  expend  to  perform  the  above  amount  of 
work,  we  find  that  the  animal  in  one  day  must  apply  1*34  Ib. ;  in 
an  hour  O'lOT  Ib. ;  and  in  a  minute  0'0028  Ib.  of  carbon,  to  the 
production  of  mechanical  effect. 

'  According  to  current  estimates,  the  work  of  a  strong  labourer 
is  -|th  of  that  of  a  horse.  A  man  who  in  one  day  lifts  1,850,000 
Ibs.  to  a  height  of  a  foot  must  consume  in  the  work  0'19  Ib.  of 
carbon.  This  for  an  hour  (the  day  reckoned  at  eight  hours) 
amounts  to  0'024  Ib. ;  for  a  minute  it  amounts  to  0-0004  Ib.  =  3'2 
grains  of  carbon.  A  bowler  who  throws  an  8-lb.  ball  with  a  ve- 
locity of  30'  consumes  in  this  effort  y^th  of  a  grain  of  carbon.  A 
man  who  lifts  his  own  weight  (150  Ibs.)  8  feet  high,  consumes  in 
the  act  1  grain  of  carbon.  In  climbing  a  mountain  10,000  feet 
high,  the  consumption  (not  taking  into  account  the  heat  generated 
by  the  inelastic  shock  of  the  feet  against  the  earth)  is  0*155  Ib.  •=  2 
ozs.  4  drs.  50  grs.  of  carbon. 

'  If  the  animal  organism  applied  the  disposable  combustible  ma- 

*  Mayer  always  uses  Centigrade  degrees. 


526  APPENDIX   TO   LECTURE   XIII. 

terial  solely  to  the  performance  of  work,  the  quantities  of  carbon 
just  calculated  would  suffice  for  the  times  mentioned.  In  reality, 
however,  besides  the  production  of  mechanical  effects,  there  is  in 
the  animal  body  a  continuous  generation  of  heat.  The  chemical 
force  contained  in  the  food  and  inspired  oxygen  is  therefore  the 
source  of  two  other  forms  of  power,  namely,  mechanical  motion 
and  heat ;  and  the  sum  of  these  physical  forces  produced  by  an  an- 
imal is  the  equivalent  of  the  contemporaneous  chemical  process. 
Let  the  quantity  of  mechanical  work  performed  by  an  animal  in  a 
given  time  be  collected,  and  converted  by  friction  or  some  other 
means  into  heat ;  add  to  this  the  heat  generated  immediately  in  the 
animal  body  in  the  same  time ;  we  have  then  the  exact  quantity 
of  heat  corresponding  to  the  chemical  processes  that  have  taken 
place. 

'  In  the  active  animal,  the  chemical  changes  are  much  greater 
than  in  the  resting  one.  Let  the  amount  of  the  chemical  processes 
accomplished  in  a  certain  time  in  the  resting  animal  be  a*,  and  in 
the  active  one  x  +  y.  If  during  activity  the  same  quantity  of  heat 
were  generated  as  during  rest,  the  additional  chemical  force  y 
would  correspond  to  the  work  performed.  In  general,  however, 
more  heat  is  produced  in  the  active  organism  than  in  the  resting 
one.  During  work,  therefore,  we  shall  have  x  plus  a  portion  of  y 
heat,  the  residue  of  y  being  converted  into  mechanical  effect. 

'  I  must  now  prove  that  the  extra  quantity  of  combustible  mat- 
ter consumed  by  the  working  animal  contains  the  necessary  force 
for  the  performance  of  the  work.  A  strong  horse,  not  working, 
is  amply  nourished  on  15  Ibs.  of  hay,  and  5  Ibs.  of  oats  per  day. 
If  the  animal  performed  daily  the  work  of  lifting  a  weight  of 
12,960,000  Ibs.  1  foot  high,  it  could  not  exist  on  the  same  nutri- 
ment. To  keep  it  in  good  condition  we  must  add  11  Ibs.  of  oats. 
The  20  Ibs.  of  nutriment  first  mentioned  is  the  quantity  which  we 
have  named  #,  and  contains,  according  to  Boussingault,  8'074  Ibs. 
of  carbon.  The  additional  11  Ibs.  of  oats,  our  quantity  y,  contains, 
according  to  the  same  authority,  4- 734. 

'  According  to  Boussiugault,  also,  the  carbon  introduced  is  to 
that  excreted  in  a  combustible  form  as  3938  : 1364-4.  Calculating 
from  these  data,  we  find  #,  or  the  quantity  of  carbon  burnt  by  the 
resting  animal,  5-2766  Ibs.,  and  y=  3'094  Ibs.  The  quantity  con- 
sumed in  mechanical  effect  is  1'34  lb.,  which  we  will  call  z. 


HAYEK   ON   VITAL   DYNAMICS.  527 

'"We  have  therefore  the  following  relations:  1.  The  mechani- 
cal effect  is  to  the  total  consumption  as  z:  x  +  y=  0*16.  2.  The 
mechanical  effect  is  to  the  surplus  consumption  of  the  working 
animal  as  z :  y  =  0'43.  3.  The  generation  of  heat  at  rest  is  to  the 
generation  of  heat  while  working  as  x:  x  +  y — z  =  0*75.' 

In  the  same  way  Mayer,  taking  the  data  furnished  by  Liebig, 
regarding  the  prisoners  and  soldiers  at  Giessen,  determines  the  fol- 
lowing relations  for  a  man :  1.  The  mechanical  effect  is  to  the  total 
consumption  as  95-7:540=0-177.  2.  The  mechanical  effect  is  to 
the  surplus  consumption  of  the  man  at  work  as  957:  285  =  0*336. 
3.  The  generation  of  heat  in  the  resting  man  to  that  in  the  work- 
ing man  255  :  540—95-7  =  0'57. 

In  these  calculations,  he  continues,  '  I  have  confined  myself  to 
the  consumed  carbon.  If  the  heat  of  combustion  be  set  equal  to 
the  carbon  +  the  hydrogen,  the  additional  heat  of  the  hydrogen 
may  be  regarded  as  nearly  =  one-fourth  of  that  of  the  carbon. 
According  to  the  individual  constitution  and  habits  of  life,  the 
labour  and  the  consumption  must  be  liable  to  considerable  varia- 
tions. The  above  results,  however,  serve  to  demonstrate  the  fol- 
lowing propositions : — 

'(1)  The  surplus  nutriment  consumed -in  the  working  organism 
completely  suffices  to  account  for  the  work  done. 

'  (2)  The  maximum  mechanical  effect  produced  by  a  working 
mammal  hardly  amounts  to  one-fifth  of  the  force -derivable  from  the 
total  quantity  of  carbon  consumed.  The  remaining  four-fifths  are 
devoted  to  the  generation  of  heat.' 

'  In  order  to  enable  them  to  convert  chemical  force  into  me- 
chanical work,  animals  are  provided  with  specific  organs,  which 
are  altogether  wanting  in  plants.  These  are  the  muscles. 

'  To  the  activity  of  a  muscle  two  things  are  necessary:  1.  The 
influence  of  the  motor  nerves  as  the  determining  condition ;  and  2. 
The  material  changes  as  the  cause  of  the  mechanical  effect. 

'  Like  the  whole  organism,  the  organ  itself,  the  muscle,  has  its 
psychical  and  its  physical  side.  Under  the  former  we  include  the 
nervous  influence,  under  the  latter  the  chemical  processes. 

'  The  motions  of  the  steamship  are  performed  in  obedience  to 
the  will  of  the  steersman  and  engineer.  The  spiritual  influence, 
however — without  which  the  ship  could  not  be  set  in  motion,  or. 


528  APPENDIX   TO   LECTUEE   XIII. 

wanting  which,  would  go  to  pieces  on  the  nearest  reef— guides,  but 
moves  not.  For  the  progress  of  the  vessel  we  need  physical  force 
— the  force  of  coal;  in  its  absence  the  ship,  however  strong  the 
volition  of  its  navigator,  remains  dead.' 

Here  follow  a  few  of  Mayer's  remarks  on  muscular  motion : — 

'  In  the  first  part  of  this  memoir,  the  part  played  by  combus- 
tion in  inorganic  apparatus  in  the  steam-engine,  for  instance,  was, 
in  its  main  characters,  explained.  Our  present  problem  is  to  con- 
sider the  phenomena  of  vitality  in  connection  with  their  physical 
causes,  and  thus  give  to  the  propositions  of  physiology  the  basis 
of  exact  science. 

'  It  has  been  already  stated  that  an  active  working  man  con- 
verts in  a  day  0'19  Ib.  of  carbon  into  mechanical  effect.  The 
weight  of  the  whole  muscles  of  such  a  man,  who  weighs  150  Ibs., 
is  64  Ibs. ;  and,  subtracting  77  per  cent,  of  water,  15  Ibs.  of  dry 
combustible  material  remains.  Let  it  be  assumed  (though  not 
granted)  that  the  heat-giving  power  of  this  mass  (with  40  per  cent, 
of  nitrogen  and  oxygen)  is  equal  to  that  of  an  equal  mass  of  pure 
carbon ;  then,  if  the  work  were  done  at  the  expense  of  the  mus- 
cles themselves,  the  whole  of  the  muscles  must  be  oxidised  and 
consumed  in  mechanical  eifect  in  eighty  days. 

'  This  arithmetical  deduction  becomes  still  more  evident  if  we 
confine  our  attention  to  the  work  performed  by  a  single  muscle — 
the  heart.  I  assume,  with  Valentin,  the  quantity  of  blood  in  the 
left  ventricle  to  be  at  every  systole  on  an  average  150  cubic  centi- 
metres. The  hydrostatic  pressure  of  the  blood  in  the  arteries  is, 
according  to  Poiseuille,  equal  to  the  pressure  of  a  column  of  mer- 
cury 16  centimetres  in  height.  The  mechanical  w^ork  done  by  the 
left  ventricle  during  a  systole  may  be  calculated  from  these  data. 
It  is  equal  to  the  raising  of  a  column  of  mercury  16  centimetres 
long,  and  with  the  base  of  a  square  centimetre,  to  a  height  of  150 
centimetres.  The  weight  of  the  mercury  amounts  to  217  grammes. 
The  mechanical  effect  of  a  systole  therefore  is — 

{325 '6  grammes  raised  1  metre, 
2  Ibs.  "      1  foot, 

which  is  equivalent  to  0'887  of  a  thermal  unit,  or  equivalent  to 
the  combustion  of  0'0001037  of  a  gramme  of  carbon.  Taking  for 
a  minute  70  strokes,  and  for  a  day  100,800  strokes  of  the  pulse, 
the  work  done  by  the  left  ventricle  in  a  day  is  equivalent  to  the 


MAYEE,   ON   VITAL   DYNAMICS.  529 

raising  of  202,000  Ibs.  to  a  height  of  one  foot.  This  is  equal  to 
89,428  thermal  units,  which  Is  equal  to  the 

comhustion  of  •}  /•  of  carbon.     According  to  Valen- 

(  168P3  grs.        ) 

tin,  the  work  done  by  the  right  ventricle  is  half  that  done  by  the 
left.  The  work  of  both  chambers  in  a  single  day  is  therefore  equal 
to  the  raising  of  303,000  Ibs.  1  foot  high  —  134,143 

thermal  units  =  \ "   '   '       £rms-  I  of  carbon. 
|  252-4  grs.       j 

1  Assuming  the  weight  of  the  whole  heart  to  be  500  grammes, 
and  deducting  from  this  77  per  cent,  of  water,  we  have  remain- 
ing 115  grammes  of  dry  combustible  material.  Assuming  this  ma- 
terial to  be  equal  to  that  of  pure  carbon,  it  would  follow  that  the 
entire  organ,  if  it  had  to  furnish  the  matter  necessary  to  its  action, 
would  be  oxidised  in  eight  days.  Taking  the  weight  of  the  two 
ventricles  alone  as  202  grammes,  under  the  same  conditions  the 
complete  combustion  of  this  muscular  tissue  would  be  effected  in  3$ 
days.' 


23 


F"-  "tor  constant  preuure,  I- ??L,  .  —,  east  of  Ireland, 

—  definition  of,  395 

-amount  of,  in  atoosphe^SDfi. 


f)  ,.     '••< 

<^> 


INDEX. 


A  BSOLUTE  zero  of  temperature,  91. 

J\_  Absorber,  qualities  necessary  to 
form  a  good,  80S. 

Absorption,  of  heat  by  coats  of  whiting 
and  tin,  303. 

elective  power  possessed  by  bod- 
ies, 313. 

takes  place  within  a  body,  318. 

by  different  thicknesses  of  glass, 

319  ;  of  selenite,  320. 

by  solid*,  Mellonf  s  table,  315. 

liquids,  ditto,  31T. 

experimental  arrangement, 

425. 

at  different  thicknesses,  ta- 
ble, 430. 

vapours  of  those  liquids,  431. 

gases,    mode    of   experiment, 

342,  et  seq. 

tables,  363,  365.  36T. 

olefiant  gas,  355,  357. 

proportional  to  density  of  gas  in 

small  quantities,  35T. 

the  transference  of  motion,  not 

annihilation,  360. 

aqueous  vapour,  39T,  et  sea.. 

435. 

a  molecular  act,  433. 

the  physical  cause  of,  437. 

from  flames,  by  vapours,   table, 

442,  443. 

—  and  radiation  of  heat,  reciprocity  of, 
809. 

by  gases  and  vapours  deter- 
mined without  external  heat,  387. 

.dynamic,  table  of  gases, 

389. 

Acoustic  experiments,  Appendix  to  Chap. 
VIII.,  296. 

Actual  energy  defined,  153. 

Aerolites,  velocity  of,  23. 

JEthrioscope.  413. 

Aggregation,  change  of  state  of,  in  bodies 
by  heat,  165. 

Air,  compressed,  chilled  by  expansion,  27. 

—  effect  of  stoppage  of  motion  of,  44. 

—  compression  of,  containing  bisulphide 
of  carbon,  43. 

-    expanded  by  heat,  80. 

—  expansion  of,  under  constant  pressure, 
81. 


Air,  expansion  of,  under  constant  volume, 
83. 

—  heated,  ascends,  illustrations  of,  ISO. 

—  cooling  effect  of,  257. 

—  passage  of  sound  through,  262. 

—  thermometer,    uninfluenced    by    heat 
that  has  passed  through  air  and  glass, 
322. 

—  not  warmed  by  passage  of  heat  through, 
324. 

—  dry,  transmission  of  heat  by,  352. 

feeble  dynamic  radiation  of,  390. 

powerful  ditto,  when  varnished  by 

vapours,  391. 

—  difficulties  in  obtaining  perfectly  pure, 
351,  397. 

—  saturated  with  moisture,  calorific  ab- 
sorption by,  402,  407. 

—  humid,  table  of  absorption  by,  at  dif- 
ferent pressures,  405. 

—  cause  of  slow  nocturnal  cooling  of,  470. 

—  distinction  between  clear  and  dry,  411. 

—  from  the  lungs,  its  calorific  absorption, 
448. 

amount  of  carbonic   acid  in, 

determined,  449. 
Alcohol,  expansion  of,  by  heat,  shown,  92. 

—  evaporation  of,  produces  cold,  172. 
Alps,  formation  and  motion  of  glaciers  on, 

202. 

Alum,  powerful  absorption  and  radiation 
of,  315. 

—  number  of  luminous  and  obscure  rays 
transmitted  by,  321. 

America,  extreme  cold  of  E.  coast  of,  197. 

Ammonia,  powerful  absorption  of  heat  by 
865 

Ancient  glaciers,  evidences  of,  207,  et  seq. 

Animal  substances,  table  of  conductive 
power  of,  246. 

Angular  velocity  of  reflected  ray  explained, 
280. 

Aqueous  vapour,  precipitated  by  rarefac- 
tion of  air,  46. 

cause  of  precipitation  of,  in  England, 

193. 

use  of,  in  our  climate,  193. 

precipitation  of  less,  east  of  Ireland, 

194. 

definition  of,  395. 

amount  of,  in  atmosphere,  396. 


532 


INDEX. 


Aqueous  vapour,  action  of,  on  radiant  heat, 
39  T,  et  seq. 

—  —  absorption  of,  in  air  obtained  from 
various  places,  404. 

objections  to  experiments  on,  an- 
swered, 404. 

cause  of  copious  precipitation  of,  in 

tropics,  407. 

effect  of  removal  of,  from  English 

atmosphere,  411. 

absorbs  same  class  of  rays  as  water, 

413. 

Asbestos,  cause  of  bad  conduction  of  heat 
by,  251. 

Asia,  cause  of  coldness  of  central  parts  of, 
411. 

Asteroids,  amount  of  heat  developed  by 
collision  of,  with  sun,  494. 

Atmosphere,  effect  of  its  pressure  on  boil- 
ing point,  181,  et  seq. 

—  diminution  of  its  pressure  lowers  boil- 
ing point,  132. 

—  amount  of  aqueous  vapour  in,  396. 

—  action  of  aqueous  vapour  in,  on  radiant 
heat,  397. 

—  use  of  aqueous  vapour  in,  41. 

—  absorption  of  solar  heat,  by,  323. 

—  the  earth's,  radiation  through,  Appen- 
dix to  Chap.  XL,  415. 

—  its  influence    on  temperature  of  the 
planets,  451. 

Atoms,  collision  of  carbon  and  oxygen,  59. 

—  when  separated,  heat  consumed,  156. 

—  enormous  attractions  of,  155. 

—  their  relative  weights,  153. 

—  possess  the  same  amount  of  heat,  158. 

—  absorb  and  emit  same  rays,  482. 
Atomic  oscillations  of  a  body  increased  by 

heat,  75. 

—  motion,  how  propagated,  79. 

—  forces,  power  of,  94. 

—  constitution,  influence  of,  on  absorption 
of  heat,  368. 

BACOjST,  extract  from  2nd  Book  of  No- 
vum  Organum,  Appendix  to  Chap. 
II.,  67. 

—  his  experiment  on  the  compression  of 
water,  155. 

Bark  of  trees,  bad  conductive  power  of, 
246. 

Bell  struck  by  hammer,  motion  not  lost, 
41. 

Beeswax,  contraction  of,  in  cooling,  120. 

Bisulphide  of  carbon,  vapour  of,  ignited 
by  compression,  43. 

note  on  compression  of  air  con- 
taining, Appendix  to  Chap.  II.,  72. 

• transparency  of,  to  heat,  313, 

317. 

Bismuth,  expansion  of,  in  cooling,  96. 

Blagden  and  Chantrey,  their  exposure  of 
themselves  in  heated  ovens,  233. 

Blood,  heat  of,  why  so  constant  in  all  cli- 
mates, 234. 

Body,  cause  of  its  resisting  high  tempera- 
tures, 234. 

Boiling  of  water  by  friction,  Eumford's 


experiments,  24,  and  Appendix  to  Chap. 

Boiling  of  water,  to  what  due,  130. 

—  point  of  water  raised  by  being  freed  of 
air,  128. 

true  definition  of,  131. 

• lowered  by  ascending,  132. 

on  summit  of  Mt.  Blanc,  Mt.  Eosa, 

&c.,  132. 

depends  on  external  pressure,  132. 

Boiler  explosions,  129, 182. 

Boracic  ether,  large  absorption  of  heat  by 
vapour  of,  374. 

table  of  dynamic  radiation  of  vapour 

of,  394 

Boutigny,  M.,  his  experiments  on  the 
spheroidal  state  of  liquids,  182. 

water  first  frozen  in  a  red-hot  cruci- 
ble by,  183. 

Brass,  expansion  of,  by  heat,  98. 

Breath,  the  human,  its  absorption  of  heat 
at  different  pressures.  448. 

a  physical  analysis  of,  449. 

Breeze,  land  and  sea,  how  produced,  191. 

British  Isles,  cause  of  dampness  of,  193. 

Bromine,  opacity  of,  to  light,  but  trans- 
parency to  heat,  proved,  370. 

Bullet,  heat  generated  by  destruction  of 
its  motion,  56. 

Bunsen,  Prof.,  description  of  his  burner, 
62. 

his  determination  of  the  tempera- 
ture of  Geysers,  140. 

his  Geyser-theory,  141. 

Bunsen's  burner,  radiation  from  flame  of, 
through  vapours,  table,  443. 


N"  of  the  galvanometer, 
\J    Mellonfs   method  of,  Appendix  to 

Chap.  X.,  376. 
Calms,  the  region  of,  192. 

—  cause  of  torrents  of  rain  in  region  of, 
409. 

Caloric  proved  not  to  exist  by  Eumford 
and  Davy,  39,  40. 

Calorific  power  of  a  body,  Eumford's  esti- 
mation of,  167. 

Calorific  conduction,  three  axes  of,  in 
wood,  245. 

of  liquids,  317. 

Candle,  combustion  of,  60. 

Capacity  for  heat,  different  in  different 
bodies,  38. 

explained,  means  of  determining, 

160. 

Carbon  atoms,  collision  of  with  oxygen, 
59. 

—  light  of  lamps  due  to  solid  particles  of, 
62. 

—  amount  of  heat  generated  by  its  com- 
bination with  oxygen,  167. 

Carbonic  acid,  how  produced  by  combus- 
tion, 60. 

solid,  properties  of,  175. 

power  of  radiation  and  absorption 

possessed  by,  363,  et  seq. 

Carbonic  oxide,  table  of  absorption  of 
heat  by,  at  different  pressures,  361. 


INDEX. 


533 


Carbonic  oxide  flame,  radiation  from, 
through  carbonic  acid  gas,  447. 

• olefiant  gas,  448. 

• human  breath,  448. 

Celestial  dynamics,  essay  by  Mayer  on, 
80,  496. 

Chan  trey  and  Blagden,  their  exposure  of 
themselves  in  heated  ovens,  233. 

Chemical  combination,  its  effect  on  ra- 
diant heat,  2S3. 

Chilling  an  effect  of  rarefaction,  44. 

—  when  produced,  277. 

—  by  radiation,  how  modified,  411. 
dew  an  effect  of,  472. 

Climate,  cause  of  dampness  of  English, 
193. 

—  mildness  of  European,  197. 

—  effect  of  aqueous  vapour  on,  192,  411. 
Clothes,  their  philosophy,  249. 
Clothing,  conductivity  of  materials  used 

in,  250. 
Clouds,  cause  of  generation  of,  408. 

—  composition  of,  199. 

Coal  mines,  cause  of  explosions  in,  255. 

Co-efficient  of  expansion  of  a  gas,  82. 

linear,  superficial,  and  cubic,  ex- 
plained, with  table,  Appendix  to  Chap. 
III.,  106. 

Cohesion,  force  of,  lessened  by  heat,  76. 

—  of  water  increased  by  removal  of  air, 
128. 

Cold,  effect  of,  on  thermo-electric  pile,  16. 

—  produced  by  rarefaction,  44. 

—  produced  by  the  stretching  of  wire,  102. 

—  of  snow  and  salt,  170. 

—  generated  in  passing  from  the  solid  to 
the  liquid  state,  170. 

from  the  liquid  to  the  gaseous 

state,  173. 
by  stream  of  carbonic  acid,  174. 

—  conduction  of,  232. 

—  apparent  reflection  of  rays  of,  286. 
Colding,  his  researches  on  the  equivalence 

of  heat  and  work,  52,  note. 

Collision  of  atoms,  heat  and  light  pro- 
duced by,  64. 

Colour,  physical  cause  of,  276. 

—  influence  of,  on  radiation,  306. 

—  of  sky,  possible  cause  of,  414. 
Combustion,  effect  of  height  on,  63. 

—  Dr.  Frankland's  memoir  on,  64. 

—  theory  of,  64. 

—  of  gases  in  tubes,  sounds  produced  by 
paper   on,  Appendix   to    Chap.  VIII., 

Compounds  good  absorbers  and  radiators, 
cause  of,  369. 

Compression,  heat  generated  by,  19. 

Compressed  air,  expansion  of,  produces 
cold,  29. 

Condensation,  congelation,  and  combina- 
tion, mechanical  value  of  each  in  the 
case  of  water,  166. 

—  effect  of,  on  specific  heat,  184. 

—  of  aqueous  vapour  in  tropics,  cause  of, 
408. 

• by  mountains,  ditto,  410. 

—  and  congelation  promoted  by  water  in 
its  different  states.  410. 


Conduction  of  heat  defined  and  illustrated, 

223. 
not  the  same  in  every  substance, 

224. 

by  metals,  225. 

experiments  of  Ingenhausz,  227. 

Despretz's  method  of  observing, 

227. 
by  different   metals  determined 

by  MM.  Wiedemann  and  Franz,  228. 

'by  crystals,  235. 

bv  wood  in  different  directions, 

table,  244. 
by  bark  of  various  trees,  table, 

246. 
importance  of  knowing  specific 

heat  in  experiments  on,  247. 

by  liquids,  256. 

by  hydrogen  gas,  257. 

—  of  cold,  illustrations  of,  232. 

—  power  of,  not  always  the  same  in  every 
direction,  235. 

Conductivity  of  metals,  table,  227. 

crystals  and  wood,  244.  et  seq. 

wood  in  three  directions,  table,  244. 

bark  of  various  trees,  ditto,  246. 

organic  structures,  ditto,  248. 

woollen  textures,  ditto,  250. 

liquids  and  gases,  256. 

Conductors,  withdrawal  of  heat  by,  253. 

—  good  and  bad,  defined,  224. 
Contraction,  generally  the  result  of  solid- 
ification, 120. 

—  of  india-rubber  by  heat,  103. 
Conservation  of  force  shown  in  steam- 
engine,  135. 

energy,  law  of,  154. 

Convection  of  heat  defined,  195. 

examples  of,  196. 

by  hydrogen,  259. 

Cooling  a  loss  of  motion,  262. 

—  effect  of  air  and  hydrogen  on  heated 
bodies,  258. 

—  how  it  may  be  hastened,  307. 
Cryophorus,  or  ice  carrier,  173. 
Crystals,  expansion  of,  102. 

—  of  ice,  124. 

—  of  snow,  200. 

—  difference  of  conductivity  in  different 
directions,  235. 

Cumberland,  traces  of  ancient  glaciers  in, 

208. 
Currents,  aerial,  how  produced,  186. 

—  upper  and  lower  in  atmosphere,  187. 


rv AVY,  Sir  H.,  his  views  of  heat,  24. 
\_J discharges  a  gunlock  in  vacuo, 

22. 
his  experiment  on  the  liquefaction 

of  ice  by  friction,  40. 
first  scientific  memoir,  Appendix 

to  Chap.  III.,  41. 
his  *  Chemical  Philosophy '  referred 

to,  46. 

investigation  of  flame,  59. 

discovery  of  the  safety  lamp,  255. 

experiment  on  the  passage  of  heat 

through  a  vacuum,  262. 


53-1 


INDEX. 


De  la  Eire  and  De  Candolle  on  conduc- 

tion of  wood,  236. 
Density,  point  of  maximum,  in  water,  94. 

—  of  gas,  relation  of  absorption  to,  356. 
Despretz,  his  experiments  on  the  conduc- 

tivity of  solids,  227  ;  of  liquids,  257. 
Dew,    Dr.  Wells'    experiments    on,  and 
theory  of,  471,  et  seq. 

—  cause  of  deposition  of,  472. 

—  a  still  night  necessary  for  the  formation 
of,  476. 

Diamond,  Newton's  opinion  of,  58. 

—  combustion  of,  in  oxygen,  59. 
Diathermancy  explained  and  illustrated, 

323. 

—  not  a  test  of  transparency,  325. 
Dilatation  of  gases  effected  without  chill- 

ing, 89. 

—  remarks  on,  Appendix  to  Chap.  TIL,  89. 
Distillation,  locomotive  force  compared  to, 

21. 
Donny,  M.,  his   experiments   on   water 

purged  of  air,  129. 

Dove,  Prof.,  quotation  from  his  work,  188. 
Drying  tubes,  difficulties  in  selecting  and 

obtaining,  851. 
Dynamic  energy  defined,  153. 

—  radiation  and  absorption,  discovery  of, 
386. 

----  of  gases,  table  of.  389. 
----  vapours,  ditto,  397. 
----  boracic  ether  vapour,  table,  893. 
----  in  different  lengths  of  tube, 
•  395. 
Dynamical  theory  of  heat,  39. 


Tjl 
FJ 

57 


AETH,  amount  of  heat  that  would  be 


generated  by  stoppage  of  its  motion, 
;  by  falling  into  the  sun,  57  ;  by  re- 
sisting the  rotation  of,  498. 

—  crust  of,  thicker  than  generally  sup- 
posed, 121. 

—  its  rotation  and  shape,  effect  of,  on  trade 
winds,  187,  190. 

—  time  required  to  cool  down,  502. 

—  all  the  energies  of,  due  to  the  sun,  503. 
Earthquake  at  Caraccas,  190. 

Elective  power  possessed  by  bodies  with 
regard  to  rays  of  light  and  heat,  312. 

Electricity  and  heat,  their  relationship 
shown  in  the  conductivity  of  various 
bodies,  228. 

—  current  of,  increased  by  cooling  con- 
ducting wire,  231. 

Elements,  bad   absorbers  and  radiators, 


Emission  theory  of  Newton,  267. 
Energy,  mechanical,  converted  to  heat,  21. 

—  potential  or  possible,  defined,  153. 

—  dynamic  or  actual,  ditto,  153. 

—  potential  and  dynamic,  sum  of,  con- 
stant, 154. 

—  all  terrestrial,  due  to  the  sun,  503. 
England,  cause  of  even  temperature  of, 

193. 
Equatorial  ocean,  winds  from,  cause  the 

dampness  of  England,  193. 
Equivalent,  mechanical,  of  heat,  54,  et  seq. 


Equivalent,  mechanical,  of  heat,  how  cal- 
culated, 84,  et  seq. 

Ether,  sulphuric,  cause  of  the  cold  pro- 
duced by  its  evaporation,  172. 

absorption  of  heat  by  vapour  of,  at 

different  pressures,  358. 

by  different  measures  of  va- 
pour of,  359. 

—  the  luminiferous  mode  of  transmission 
of  light  and  heat  by,  268. 

fills  all  space  and  penetrates  all 

bodies,  311. 

the  power  of  imparting  motion 

to,  and  accepting  motion  from,  are  pro- 
portional, 308,  317,  864. 

Euler,  his  argument  for  the  undulatory 
theory  of  light,  267. 

Europe  the  condenser  of  the  Atlantic, 
198. 

—  cause  of  mildness  of  climate  of,  197. 
Evaporation  produces  cold,  172. 

—  water  frozen  by,  173. 
Exchanges,  Prevost's  theory  of,  278. 
Expansion  of  volume,  75. 

gases  by  heat,  80. 

co-efficient  of,  82. 

without  performing  work,  89. 

liquids  by  heat,  92. 

water  in  freezing,  94. 

use  of,  in  Nature,  95. 

alcohol  by  heat,  92. 

water  by  heat,  93. 

cold,  94. 

bismuth  in  cooling,  96. 

solid  bodies  by  heat,  97. 

lead,  curious  effect  of,  100. 

crystals,  101. 

Expansive  force  of  heat,  79. 
Explosions  of  steam-boilers,  128. 

possibly  due  to  the  spheroidal 

state,  182. 

—  in  coal  mines,  cause  of,  255. 
Extra-red  and  extra-violet  rays,  273. 


T71AEADAY,  his  discovery  of  magneto- 
JD     electricity,  50. 

experiments  on  melted  ice,  129. 

discovery  of  the  relegation  of  ice, 

203. 

—  mercury  first  frozen  in  a  red-hot  cruci- 
ble by,  183. 

Fibre  of  wood,  power  of  conduction  of 

heat  by,  244 
Fire  produced  by  friction,  22. 

—  syringe,  43. 

—  balloon,  81. 

—  screens  of  glass,  action  of,  323. 
Flame,  constitution  of,  59,  et  seq. 

—  cause  of  its  inability  to  pass  through 
wire  gauze,  254. 

Flames,  singing,  paper  on,  Appendix  to 

Chap.  VIII.,  288. 
Count   Schaffgotsch's    experiments 

on,  Appendix  to  Chap.  VIII.,  286. 

—  examination  of  radiation  from,  441,  et 
seq. 

Fluorescence  of  sulphate  of  quinine  in  the 
invisible  spectrum,  273. 


INDEX. 


535 


Foot-pounds,  explanation  of,  54, 

Forbes,  Prof.  J.  D.,  his  viscous  theory  of 
ice,  207. 

law  of  movement  of  glaciers,  Ap- 
pendix to  Chap.  VI.,  212. 

Force  of  heat  in  expanding  bodies,  99. 

—  vital,  supposed  conservative  action  of, 
234. 

Forces,  molecular,  energy  of,  94, 155. 

—  polar,  heat  required  to  overcome,  1 64. 
Frankland,  Dr.,  his  experiments  on  com- 
bustion, 63. 

Fraunhofer's  lines,  4S5. 

Freezing,  effect  of,  on  water  pipes,  101. 

—  point  lowered  by  pressure,  125. 

—  of  water  produced  by  its  own  evapora- 
tion, 173. 

—  together  of  pieces  of  ice,  202. 
Friction,  generation  of  heat  by,  1 8. 

—  against  space,  heat  developed  by,  48. 
Frost,  means  of  preserving  plants  from, 

474. 

—  cause  of  their  preservation,  474. 
Fusible  alloy  liquefied  by  rotation  in  mag- 
netic field,  51. 

Fusion,  point  of,  effect  of  pressure  on,  121. 


|~1  ALVANOMETER  described,  15. 
\JC    —  note  on  the  construction  of,  Ap- 
pendix to  Chap.  L,  32.      = 

—  peculiarity  of,  in  high  deflections,  346. 

—  Mellonfs  method  of  calibrating,  Ap- 
pendix to  Chap.  X.,  376. 

Gas,  carbonic  acid,  liberation  of,  from  so- 
da-water, consumes  heat,  28. 

—  combustion  of,  60. 

—  illuminating  power  of,  63. 

—  co-efficient  of  expansion  of,  82. 

—  absorbs  those  rays  which  it  emits,  482. 

—  radiation  from  a  luminous  jet  of,  442. 
Oases,  constitution  of,  77. 

—  velocity  of  particles  of,  104. 

—  expansion  ot',  by  heat,  79. 

—  specific  heat  of,  simple  and  compound, 
162,  et  seq. 

—  conductivity  of,  256. 

—  first  experiments  on  their  absorption 
of  heat,  342. 

—  mode  of  experiment  improved,  849. 

—  different  powers  of  accepting  motion 
from  the  ether,  or  difference  in  absorp- 
tion possessed  by,  355. 

—  different  powers  of  imparting  motion 
to  the  ether  or  difference  in  radiation 
possessed  by,  362. 

—  table  of  dynamic  radiation  of,  372. 
Gaseous  condition  of  matter,  76. 
Gassiot,  iron  cylinders  burst  by,  95. 
Gauze  wire,  cause  of  its  stopping  passage 

of  flame,  254. 

Geyser,  the  Great,  of  Iceland,  description 
of,  135. 

—  Bnnsen's  theory  of,  138. 

—  produced  in  lecture  room,  141. 

—  its  history,  142. 
Glaciers,  formation  of,  202. 

—  motion  of,  described,  202. 

«-  point  of  swiftest  motion  shifts,  202. 


Glaciers,  their  daily  rate  of  motion,  202. 

—  viscous  theory  of,  202. 

—  regelation  ditto,  203. 

—  ancient,  evidences  of,  in  various  places, 
207,  et  seq. 

—  hypotheses  to  account  for,  209,  et  seq. 

—  cold  alone  cannot  produce,  210. 

—  their  laws  of  movement  established, 
Appendix  to  Chap.  VI.,  212. 

Glaisher,  his  table  of  nocturnal  radiation, 

475. 
Glass,  why  cracked  by  hot  water,  99. 

—  broken  by  a  grain  of  quartz,  100. 

—  opacity  of,  to  heat,  314. 

—  absorption  of  heat  by  different  thick- 
nesses of,  319. 

—  fire-screens,  use  and  philosophy  of,  323. 
Gmelin,  his  definition  of  heat,  37. 

Gore,  hla  experiments  on  revolving  balls, 

118. 
Gravity,  velocity  imparted  to  a  body  by, 

56. 
Grease,  philosophic  use  of,  on  wheels  and 

axles,  21. 
Gulf-stream,  19T. 
Gypsum,  powdered,  bad  conduction  of 

heat  by,  252. 


TTARMONICA,  chemical,  297. 

XI    Heat  and  cold,  opposite  effects  upon 

thermo-electric  pile,  16. 
Heat,  generated  by  mechanical  processes^ 

-  —  friction,  18. 

compression,  19. 

percussion,  19. 

falling  of  mercury  or  water,  20. 

—  consumption  of,  in  work,  26. 

—  nature  of,  37,  et  seq. 

—  a  motion  of  ultimate  particles.  39. 

—  considered  tlms,  by  Locke  39. 

Bacon,  39. 

Rumford,  23. 

Davy,  41. 

—  developed  when  air  compressed,  41. 

motion  of  air  stopped,  28,  45. 

by  rotation  in  magnetic  field,  51. 

—  mechanical  equivalent  of,  54,  84,  et  seq. 

—  proportional  to  height  through  which 
a  body  falls,  55. 

—  relation  of,  to  velocity,  56. 

—  an  antagonist  to  cohesion,  75. 

—  of  friction,  Rumford's    essay  on  the 
source  of,  Appendix  to  Chap.  II.,  68. 

—  expansion  of  gases  by,  80, 

liquids  by,  92. 

solids  by,  87. 

—  imparted  to  gas  under  constant  pres- 
sure, 81. 

at  constant  volume,  88. 

—  produced  by  stretching  india-rubber, 

—  direct  conversion  into  mechanical  mo- 
tion, 113. 

—  developed  by  electricity,  118,  230. 

—  performance  of  work  by,  in  steam  en- 
gine, 134. 

—  power  of,  in  expanding  bodies,  156. 


536 


INDEX. 


Heat,  two  kinds  of  motion  produced  in 
bodies  by,  157. 

—  interior  work  performed  by,  157. 

—  consumed  in  forcing  atoms  asunder, 
157. 

—  generated  by  atoms  falling  together, 
158. 

—  quantity  yielded  up  by  different  bodies 
in  cooling,  159. 

—  specific,  160,  et  seg. 

—  causes  change  of  state  of  aggregation 
in  bodies,  165. 

—  latent,  of  water,  steam,  and  aqueous 
vapour,  166,  et  seq.,  210. 

definition  of,  1 66. 

—  generated  in  passing  from   liquid  to 
Bolid  state,  170. 

—  cause  of  more  equal  distribution  of, 
193, 197. 

—  connection  of,  1 97. 

—  necessary  for  the  production  of  glaciers, 
210. 

—  distinction   between  it  and   ordinary 
motion,  222. 

—  conduction  of,  defined  and  illustrated, 
223. 

not  eqnal  in  every  substance,  224. 

—  method  of  determining  the  conducti- 
bility  of  bodies  for,  227. 

—  and  electricity,  relationship  of,  229. 

—  motion  of,  interferes  with  the  motion 
of  electricity,  230. 

—  conversion  of,  into  potential  energy, 

—  difference  of  conductivity  of,  in  crys- 
tals and  wood,  235,  et  seq. 

—  transmission    of,  through    -wood,  237, 
244. 

influenced    by   the    mechanical 

state  of  the  body,  251. 

—  doubtful  conduction  of,  by  hydrogen 
gas,  259. 

—  its  passage  through  a  vacuum,  262..  •-— 

—  analogy  of,  to  sound,  267. 

—  to  what  motion  of,  imparted,  269. 

—  radiant,  269. 

—  rays  beyond  visible  spectrum,  272. 

—  obeys  the  same  laws  as  light.  2S1. 

—  action  of  an  oxygen  and  hydrogen,  285. 

—  law  of  inverse  squares  applied  to,  302. 

• — transversal  undulation  of  waves  of,  304. 

—  quality  of,  821. 

• —  transmission  of,  through  opaque  bodies, 
325. 

—  effect  of,  on  ice,  328. 

—  absorption  of,  by  gases,  first  mode  of 
experiment,  342. 

means  of  detecting  minute 

amount  of,  346. 

—  absorption  of,  by  gases,  improved  appa- 
ratus for  researches  on,  described,  349. 

-  -  free  passage  of.  through  dry  air,  oxy- 
gen, hydrogen,  and  nitrogen,  352. 

—  tables  of  absorption  of,  by  gases  and 
vapours,  849,  et  seq. 

—  spectrum,  detached  from  luminous.  373. 

—  absorption  and  radiation  of  a  gas  or 
vapour   determined   "without   external 
heat,  354. 


Heat,  absorption  of,  by  aqueous  vapour, 
896,  et  seq. 

—  amount  of,  generated  by  collision  of 
meteors  with  the  sun,  492. 

—  developed  by  friction  of  tidal  wave, 
496. 

—  source  of  this  heat,  496. 

Height,  influence  of,  on  combustion,  63. 
Helmholtz,  his  calculation  of  the    heat 

that  would  be  developed  by  stoppage  of 

earth's  motion,  57. 
remarks  on  the  exhaustion  of  the 

mechanical  force  of  our  system,  499. 
Herschel,  Sir  William,  his  discovery  of 

the  obscure  rays  of  the  spectrum,  272. 

—  Sir  John,  note  on  rocksalt,  344. 
measurements  of  solar  radiation, 

487. 

Herbs,  aromatic,  action  of  their  odours 
on  radiant  heat,  880. 

Humboldt  on  the  cold  of  Central  Asia, 
411. 

Huyghens,  his  theory  of  light,  26T. 

Hydrogen,  collision  of  atoms  of,  with  oxy- 
gen, 60. 

—  amount  of  heat  generated  by  combin- 
ing with  oxygen,  to  form  water,  1P>7. 

—  cooling  effect  of,  on  heated  bodies?,  258. 

—  low  power  of  absorption  possessc  d  by, 
363,  et  seq. 

character*  of  its  radiation,  45'/,  and 

Appendix  to  Chap.  XII.,  456. 


ICE  liquefied  by  friction,  40,  and  Appen- 
dix to  Chap.  III.,  110. 

—  why  it  swims  on  water,  94. 

—  liquefied  by  pressure,  122. 

—  structure  and  beauty  of,  123. 

—  dissected  by  beat,  124. 

—  flowers,  124,  et  seq. 

—  extracts  from  memoir  on  physical  prop- 
erties of,  Appendix  to  Chap.  IV.,  148, 
and  to  Chap.  IX.,  328. 

—  carrier,  or  cryophorus,  173. 

—  viscous  theory  of,  202. 

—  regelation  ditto,  203. 

—  moulded  by  pressure,  204. 

— artificial  formation  of,  by  nocturnal  ra- 
diation, 474. 

—  this  theory  supplemented,  475. 

—  amount  melted  per  minute  by  solar  ra- 
diation, Herschel  and  Pouillet's  meas- 
urements, 487. 

—  amount  melted  per  hour  by  total  emis- 
sion of  sun,  490. 

Iceland,  geysers  of,  136. 

India-rubber,  stretching  of,  produces  heat, 
102. 

contraction  of,  by  heat,  103. 

Ingenhausz,  his  experiments  on  the  con- 
duction of  heat,  227. 

Interior  work  performed  by  heat,  157. 

different  kinds  of,  169. 

Iodine  dissolved  in  bisulphide  of  carbon, 
diathermancy  of,  870. 

Ireland,  more  rain  on  west  Bide  than  on 
east,  194. 

—  traces  of  ancient  glaciers  in,  208. 


IIsDEX. 


537 


Iron  bottle  burst  by  freezing  water,  94. 

—  expansion  of,  by  heat,  98. 

—  presence  of,  in  sun,  proved.  4^6. 
Isothermal  line  runs  north  and  south  in 

England,  197. 
Ivory,  bad  conductivity  of,  246. 


JOULE,  Dr.,  his  experiments  on  the 
mechanical  equivalent  of  heat,  25, 
87,  et  aeq. 

heat  and  work,  52. 

magneto-electricity,  87. 

• the  shortening  of  india-rub- 
ber by  heat,  102. 

explains  heat  of  meteorities,  23. 

his  experiments  on  the  cold  pro- 
duced by  stretching  wires,  102. 

—  extract  from  a  paper  by,  Appendix  to 
Chap.  XIII.,  522. 


T7-NOBLAUCH,  explanation  of  some  of 

IV    his  results,  453. 

Kopp,  Professor,  his  determination  of  the 
cubic  co-efficients  of  expansion,  Appen- 
dix to  Chap.  III.,  107. 


T  AMPBLACK,  anomalous  deportment 
J_J    of,  370. 

—  radiation  of  heat  through,  371. 
from,  441. 

Land  and  sea  breezes,  how  produced,  191. 

Latent  heat  of  water,  40,  166. 

•  — •  mechanical  value  of,  168. 

liquids,  169. 

vapours,  172. 

Lead  ball  heated  by  collision,  55. 

—  curious  effect  of  expansion  of,  100. 
Leidenfrost,  first  observer  of  the  sphe- 
roidal state  of  liquids,  181. 

Leslie's  cube,  radiation  from,  441. 

—  sethrioscppe,  412. 

—  observations  explained,  412. 

Light  produced  by  friction  of  quartz,  23. 

—  of  lamps,  to  what  due,  60. 

—  of  pras  destroyed  when  mixed  with  air, 

62. 

—  law  of  diminution  with  distance,  302. 

—  theories  of,  267. 

—  analogy  of  sound  to,  267. 

—  propagation  and  sensation  of,  268. 

—  reflection  of,  279. 

—  action  of,  on  chlorine  and  hydrogen, 
284. 

—  undulations  of  transversal,  304. 
Liquefaction  of  ice  by  friction,  40. 

pressure,  122. 

Liquid  condition  of  matter,  76. 

—  changing  to  solid  produces  heat,  167. 
Liquids,  expansion  of,  by  heat,  92. 

—  the  spheroidal  state  of,  173. 

—  conductivity  of,  256. 

—  calorific  transmission  of,  Melloni's  ta- 
ble, 317. 

—  apparatus  for  determining  their  absorp- 
tion of  heat  at  different  thicknesses,  424. 

•—  table  of  absorption  of  neat,  by,  430. 

23* 


Liquids  and  their  vapours,  order  of  their 
absorption  of  heat,  432,  435. 

Lloyd,  Dr.,  his  tables  of  rainfall  in  Ire- 
land, 194, 

Locke,  hig  view  of  heat,  39. 

Luminous  and  obscure  radiation,  827.  and 
Appendix  to  Chap.  XII.,  456. 


MAGNUS,  Professor,  his  experiments 
on  gaseous  conduction.  257. 
the  conductivity  of  hydro- 
Ken,  259. 

Magnetic  field,  apparent  viscosity  of,  49. 
Material  theory  of  heat,  37. 
Matter,  liquid  condition  of,  76. 

—  gaseous  ditto,  76. 

Mayer,  Dr.,  compares  locomotive  force  to 
distillation,  52. 

enunciates  the  relationship  between 

heat  and  work,  52. 

his  calculation  of  the  heat  that  would 

be  produced  by  stoppage  of  earth's  mo- 
tion, 57. 

mechanical   equivalent   of  heat, 

essay  on  celestial  dynamics,  re- 
ferred to,  496. 

meteoric  theory  of  sun's  heat,  496. 

extracts  from  his  paper  on  organic 

motion,  Appendix  to  Chap.  XIII.,  52-2. 

Maximum  density  of  water,  93. 

Mechanical  processes,  generation  of  heat 
by,  17. 

—  work,  consumption  of  heat  in,  26. 

—  theory  of  heat,  26. 

—  equivalent  of  heat,  54. 

Mayer's  determination,  52,  86. 

Joule's  determination,  52,  87. 

Meidinger,  M.,  his  experiments  on  ozone, 
384,  note. 

Mclloni,  his  mode  of  proving  the  diminu- 
tion of  heat  as  the  square  of  the  dis- 
tance, 303. 

—  his  researches  on  radiant  heat,  314. 

—  his  table  of  the  transmission  of  heat 
through  solids,  315. 

table  of  the  transmission  of  heat 

through  liquids,  317. 

—  source  of  error  in  his  experiments  on 
transmission  of  heat  through  liquids, 
424. 

—  his  theory  of  serein,  413. 

—  explanation  of  some  of    his   results, 
455 

—  his  addition  to  the  theory  of  dew,  477. 
experiments  on  the  warmth  of  the 

lunar  rays,  477. 
Mercury,  low  specific  heat  of,  159. 

—  frozen  by  solid  carbonic  acid,  175. 

in  red-hot  crucible,  183. 

Mer-de-Glace,  abstract  of  discourse  on, 

Appendix  to  Chap.  VI.,  212. 
Metals,  good  conductors  of  heat,  225. 

—  bad  radiators;  805. 
absorbers,  310. 

—  effect  of  their  bad  radiation,  473. 

—  bands  seen  in  spectra  of  their  vapours, 
480. 


538 


INDEX. 


Metals,  presence   of  terrestrial,  in  sun, 

proved,  486. 
Meteors,  zodiacal  light  supposed  to  be,  58, 

493. 
• —  number  of,  seen  in  Boston,  492. 

—  amount  of  heat  generated  by  collision 
of,  with  sun,  494. 

—  sun's  light  and  heat  possibly  kept  up 
by,  492. 

Meteorology,  absorption  of  heat  by  aque- 
ous vapour  applied  to  phenomena  of, 
407,  e t  seq. 

Mitscherlich,  Professor,  his  experiments 
on  the  expansion  of  crystals,  101. 

Molecular  motion,  heat  defined  as,  41,  75, 
222. 

—  vibration  of  a  body  more  intense  when 
heated,  75. 

—  forces  irresistible,  94. 

power  of,  156. 

calculated,  161. 

—  action  in  wood,  effect  of,  245. 
Mongolfier,  equivalence  of  heat  and  work, 

maintained  by,  52. 
Moon-blindness,  cause  of,  474. 

—  beams,  cause  of  their  putrefying  power, 
474. 

Moon,  experiments  on  warmth  of,  478. 

—  obscure  heat  of,  cut  off  by  our  atmos- 
phere, 478. 

Mosely,  Eev.  Canon,  curious  effect  of  ex- 
pansion noted  by,  79. 

Motion,  heat  considered  to  be,  by  Bacon 
and  Locke,  39;  by  Eumford,  25;  by 
Davy,  24. 

—  transference  of,  from  mass  to  molecules, 
75. 

—  point  of  maximum  in  a  glacier,  214. 
Mountains,  their  action  as  condensers,  ex- 
plained, 408.      . 

Moving  force,  amount  of  heat  produced 
by  destruction  of,  57. 

produced  by  steam,  135. 

or  dynamic  energy,  defined,  153 


"VTATITEE,  adaptation  of  means  to  ends 
JM      in,  116. 

Natural  philosopher,  his  vocation,  116. 
Neve,  the  feeder  of  the  glacier,  202. 
Newton,  his    opinion   of    the   diamond, 

58. 

theory  of  light,  267. 

Nitrogen,  velocity  of  particles  of,  78. 
Nitrous  oxide,  absorption  and  radiation 

of,  364. 

dynamic  radiation  of,  389. 

—  acid  gas,  bands  produced  by  spectrum 

of,  482. 
Nocturnal  radiation,  artificial  formation 

of  ice  by,  476. 
experiments  on,  by  "Wells,  Glaisher, 

and  others,  474,  477. 
Novum  Organum,  extract  from  2nd  book 

of,  Appendix  to  Chap.  II.,  67. 


0 


IBSCtTEE  heat,  rays  of,  obey  same  laws 
as  light,  281? 


Obscure  heat,  ratio  of,  to  luminous  rays 
from  different  sources.  327,  and  Appen- 
dix to  Chap.  XII.,  456.  • 

Ocean,  influence  of,  on  temperature,  164. 

Olefiant  gas,  athermancy  of,  353. 

table  of  "bsorption  by  at  different 

pressures,  355. 

by  various  measures,  357. 

radiation  of,  364. 

dynamic  radiation  of,  389. 

varnishing  metal  by,  390. 

Organic  motion,  extracts  from  a  paper  by 
Mayer  on,  84,  and  Appendix  to  Chap. 
XliL,  522. 

—  structures,  table  of  conductivity  of,  246. 
Oxvgen,  collision  of  atoms  of  with  carbon, 

59. 

—  velocity  of  particles  of,  78. 

—  small  absorption  of  heat  by,  364. 
Ozone,  action  of,  on  radiant  heat,  382. 

-—  increase  of,  by  reduction  in  size  of  elec- 
trodes. 3S3. 

—  probable  constitution  of,  385. 

PAETICLES,  impact  of,  causes  of  sen- 
sation of  heat,  78. 

—  ultimate,  motion  of,  produces  heat,  39. 
Parabolic  mirrors,  reflection  of  light  and 

heat  from,  283,  et  seq. 
Percussion,  heat  generated  by,  20. 
Perfumes,  how  propagated,  77. 

—  table  of  absorption  of  heat  by,  380. 
Period,  heat  and  light  differ  only  in,  437. 

—  influence  of.  on  Absorption,  437. 

—  determines  the  quality  of  heat  emitted 
by  bodies,  438. 

Periods,  vibrating,  of  formic  and  sulphuric 

ether,  441. 

of  a  hydrogen  flame,  452. 

shortening  of,  452. 

Perspiration,  use  of,  in  hot  climates,  234. 
Photosphere  of  sun,  action  of,  on  solar 

rays,  485. 
Phvsical  analysis  of  the  human  breath, 

449. 

—  properties  of  ice,  extracts  from  a  me- 
moir on,  Appendix  to  Chap.  IV.,  148, 
and  to  Chap.  IX.,  328. 

—  basis  of  solar  chemistry,  a  lecture  on, 
Appendix  to  Chap.  XIII.,  516. 

Pile,  thermo-electric,  construction  and  use 
of,  14,  and  Appendix  to  Chap.  I.,  30. 

Pitch  of  note,  upon  what  dependent,  275. 

Planets,  orbital  velocity  of  the  inferior, 
23. 

—  heat  that  would  be  developed  by  their 
falling  into  sun,  or  by  resisting  rotation 
of,  499. 

Platinum  lamp,  described,  427. 

Polar  forces,  heat  required  to  overcome, 
164. 

Potential  or  possible  energy  defined,  153. 

Pouillet,  M.,  his  experiments  on  the  tem- 
perature of  air  and  swan's  down,  477. 

measurement  of  solar  radiation, 

487 ;  of  its  partial  absorption  by  our  at- 
mosphere, 489. 

pyrhcliometer,  487. 


INDEX. 


539 


Pressure,  relating  to  heating  of  gases,  81, 
80. 

—  effect  of,  on  point  of  fusion,  121. 
crust  of  earth,  121. 

—  liquefaction  of  ice  by,  123. 
Prevost's  theory  of  exchanges,  277. 
Pyrheliometcr,  use  and  description  of,  4S7. 
Pyrometers,  98. 


QUARTZ,  clear  and  smoky,  transmit 
equal  amounts  of  heat,  316. 
quality  of    radiant   heat,    definition   of, 


T>  ADIANT  heat,  definition  of,  269. 

Xv and  light,  analogy  between, 

269. 

emitted  by  all  bodies,  277. 

laws  the  same  as  those  of  light, 

280. 

reflection  and  convergence  of  rays 

of,2S2. 

law  of  inverse  squares  applied  to, 

303,  et  seq. 

its  origin  and  propagation,  304 

apparatus .  for  researches  on,  de- 
scribed, 349.  " 

absorption  of,  by  gases,  366. 

vapours,  372. 

action  of  perfumes  on,  379. 

— .—  object  of  researches  on,  423. 

Radiation,  effect  of  colour  on,  306. 

Radiation  and  absorption,  reciprocity  of, 
308. 

—  of  metals,  805. 

heat  by  solids,  305. 

gases,  364. 

vapours,  372. 

—  and  absorption  of  a  gas  or  vapour  de- 
termined without  external  heat,  387. 

dynamic,  table  of  gases,  389. 

•  —  dew,  an  effect  of  chilling  by,  472. 

—  nocturnal,  Glaisher's  table  of  chilling 
by,  476. 

artificial  formation  of  ice  by,  474. 

—  through  the  earth's  atmosphere,  Ap- 
pendix to  Chap.  XL,  401. 

—  luminous  and  obscure,  327,  and  Appen- 
dix to  Chap.  XII.,  456. 

Radiating  body  and  air,  difference  be- 
tween constant,  477. 

Eain,  cause  of  the  torrents  of,  in  the  trop- 
ics, 192. 

—  fall,  greater  on  west  than  on  east  coast 
of  Ireland,  193. 

Dr.  Lloyd's  table  of,  in  Ireland,  193. 

—  —  place  where  the  greatest  occurs,  194, 
note. 

— " —  upon  what  dependent,  194. 
Rarefaction,  chilling  effect  of,  44. 

—  will  not  by  itself  lower  mean  tempera- 
ture, 89. 

Rectilinear  motion,  atoms  of  gases  move 

with,  77. 

Refrigeration  by  expansion  of  a  gas,  86. 
Reflection  of  light  and  heat  obey  same 

laws,  278. 


Regelalion,  discovery  of,  by  Faraday,  203. 

—  of  snow  granules,  note  on,  Appendix  to 
Chap.  VI.,  220. 

Rendu,  his  plastic  theory  of  ice,  207. 

Resistance,  heat  of  electric  current  pro- 
portional to,  119. 

Revolving  balls,  Gore's  experiments  on, 
119. 

Rifle  ball,  amount  of  heat  generated  by 
stoppage  of  its  motion,  56. 

Rocker  used  in  the  Trevelyan  instrument. 
115. 

Rocksalt,  transparency  of,  to  heat,  314. 

—  use  of,  in  experiments  on  absorption  of 
heat  by  gases,  342. 

—  hygroscopic  character  of,  399. 

—  deposition  of  moisture  on,  avoided,  400. 

—  cell,  described,  425. 

Rumford,  Count,  his  experiments  on  heat 
produced  by  friction,  23,  and  Appendix 
to  Chap.  II.,  68. 

overthrow  of  the  material  theory 

of  heat,  89. 

abstract  of  his  essay  on  heat,  Ap- 
pendix to  Chap.  II.,  68. 

his  estimation  of  the  calorific  power 

of  a  body,  166. 

experiments  on  the  conductivity 

of  clothing,  250. 

liquids  and  gases, 

256. 

transmission  of  heat 

through  a  vacuum,  262. 

Rupert's  drops,  100. 


SAFETY  LAMP,  explanation  and  use 
of,  255. 

Bait   and    sugar,  dissolving  of,  produces 
1      cold,  65. 

—  common,  yellow  bands  emitted  and  ab- 
sorbed by  vapour  of,  484. 

Scents,  action  of,  on  radiant  heat,  379. 

Schemnitz,  machine  for  compression  of  air 
at,  46. 

Schaffgotsch,  Count,  his  paper  on  acoustic 
experiments,  Appendix  to  Chap.  VIII., 
291. 

Schwartz,  his  observation  of  sound  pro- 
duced by  cooling  silver,  114. 

Sea  warmer  after  a  storm,  20, 

—  breeze,  how  produced,  191. 

Seguin,  equivalence  of  heat  and  work  de- 
veloped by,  52. 

Selenite.  absorption  of  heat  by  different 
thicknesses  of,  320. 

Senarmount,  his  experiments  on  the  con- 
duction of  heat  by  crystals,  235. 

Serein,  Melloni's  theory  of,  413. 

Shooting  stars,  theory  of,  23. 

Silica,  water  of  Geysers  contains  and  de- 
posits, 136. 

—  as  crystal,  high  conductive  power  of, 
246. 

—  as  powder,  low  ditto,  251. 
Singing  flames,  265. 

paper  on,  Appendix  to  Chap.  VIIL. 

288. 
Sky,  colour  of,  414. 


54:0 


INDEX. 


Snow,  shower  of,  produced  by  issuing  of 
compressed  air,  46. 

—  carbonic  acid,  174. 

—  crystals,  200. 

—  lino,  the,  201. 

—  formation  of  glaciers  from,  201. 

—  ball,  cause  of  coherence  of,  204. 

—  bridges,  how  crossed,  205. 

—  squeezed  to  ice,  205. 

—  granules,  note  on  the  regelation  of,  Ap- 
pendix to  Chap.  VI.,  217. 

Sodium,  yellow  bands  emitted  and  ab- 
sorbed by  vapour  of,  484,  487. 

Solar  spectrum,  cause  of  dark  lines  in, 
436. 

—  chemistry,  a  lecture  on  the  physical 
basis  of,  Appendix  to  Chap.  XIIL,  516. 

Solids,  expansion  of,  by  heat,  98. 

—  calorific  transmission  of,  Melloni's  ta- 
ble, 315. 

Solidification  accompanied  by  expansion, 
94. 

contraction,  120. 

Sound,  conversion  of  heat  into,  115. 

—  mode  of  its  transmission  through  air, 
264. 

—  produced  by  flame,  265. 

—  undulation  of  waves  of,  longitudinal, 
304. 

—  analogy  of  heat  and  light  to,  268. 
Sounds,  inaudible,  277. 

—  musical,  produced  by  gas  flame  in  tubes, 
265,  and  Appendix  to  Chap.  VIII.,  238. 

Specific  heat  of  bodies,  how  determined, 

160. 

elementary  bodies,  160. 

simple  and  compound  gases,  163, 

et  seq. 
of  water  the  highest  consequences 

attending,  164. 
masking  the  conductive  power  of  a 

body,  247. 

Spectra  of  zinc,  copper,  &c.,  481, 
Spectrum,  heat  of  non-luminous,  proved, 

272. 

—  non-luminous,  obtained,  373. 

—  solar,  cause  of  dark  lines  in,  486. 

—  of  incandescent  carbon,  479. 

—  of  solids,  similar  to,  479.  , 
Spheroidal  state  of  liquids,  176,  et  seq. 

—  condition,  first  observer  of,  181. 
Spheroid,  floating  of,  on  its  vapour,  176. 

—  not  in  contact  with  vessel,  proved,  179. 
Spiral  of  platinum  wire  heated  by  electric 

current,  radiation  from,  440,  452. 
Springs,  boiling,  of  Iceland,  described,  136. 
Steam,  how  produced,  130. 

—  elastic  force  of,  increased  by  heating, 
134. 

—  latent  heat  of,  166. 

Storms  produced  by  heated  air,  186. 
Strokkur,  the,  imitation  of,  141. 
Sulphate  of  soda,  cold  produced  by  dis- 
solving, 170. 
heat  produced   by  crystallising, 

Sulphuric  acid  used  for  drying  gases,  354. 

—  ether,  absorption  of  heat  by  vapour  of, 
858,  372. 


Sun,  probable  cause  of  continuance  of 
heat  and  light  of,  58,  492. 

—  production  of  winds  by  heat  of,  186. 

—  does  not  heat  dry  air  sensibly,  323. 

—  constitution  of,  485. 

—  flame  atmosphere  surrounding,  485. 

—  and  planets,  supposed  common  origin 
of,  486. 

—  heating  'power  of,  measurements   by 
Ilerschel  and  Pouillct,  487. 

—  mode    of    determining    the    radiation 
from,  488. 

—  atmospheric  absorption  of  heat  of,  489. 

—  total  amount  of  heat  emitted  by,  490. 

—  all  organic  and  inorganic  energy  re- 
ferred to,  503. 

—  small  fraction  of  its  heat  that  produces 
all  terrestrial  energy,  514. 

Switzerland,  evidences  of  ancient  glaciers 
in,  207. 


mEMPEEATUEE,    absolute    zero    of, 
JL     113. 

—  influence  of,  on  conduction  of  electri- 
city, 230. 

—  high,  how  endured,  233. 

—  dew  caused  by  lowering  of,  472. 

—  influence  of,  on  the  quality  of    heat 
emitted  by  a  body,  518. 

on  transmission,  522. 

—  difliculties  in   ascertaining   the  true, 
473. 

Teneriffe,  Peak  of,  two  currents  blow  on. 

215. 
Thermal  effects  produced  by  stoppage  of 

motion,  43,  54. 
Thermo-electric  pile,  14. 
note  on  the  construction  of,  Ap- 
pendix to  Chap.  I.,  80. 
used   in   researches   on   radiant 

heat,  348. 
Thermometer,  construction  of,  Appendix 

to  Chap.  III.,  107. 
Thickness,  influence  of,  on  absorption  of 

heat,  319. 
Thomson,  Professor  William,  on  earth's 

crust,  95. 
his  suggestion  that  india-rubber 

would  shorten  by  heat,  103. 
meteoric   theory  of  the  sun, 

57. 
tables  of  energy,  498. 

—  Professor  James,  on  the  influence  ot 
pressure  on  fusion,  122. 

Tidal  wave,  velocity  of  earth's  rotation 

diminished  by,  496. 
Trade  winds,  upper  and  lower,  187. 
Transparency  of  bodies,  cause  of,  311. 

—  not  a  test  for  diathermancy,  325. 
Transmission  of  heat  through  solids,  Mel- 

loni's  table,  315. 

liquids,  ditto,  317. 

influence  of  temperature  of  source 

on,  315,  317. 

Trevelyan,  Mr.  A.,  his  instrument,  114. 

cause  of  vibrations  of,  117. 

abstract  of  a  lecture  on,  Ap- 
pendix to  Chap.  IV.,  144. 


I3DEX. 


541 


Tropics,  flow  of  air  from  and  to,  187. 

—  the  region  of  calms  or  rains,  192. 

—  cause  of  the  torrents  of  rain  in,  401. 


TTNDTTLATION  TIIEOEY,  267. 


TTACUUM  in  centre  of  ice  flowers,  127. 
y  _  passage  of  heat  through,  262. 

—  dry  air  similar  to,  with  regard  to  ra- 
diant heat,  852. 

Vapour  of  water  condensed  by  rarefaction 

of  air,  45. 
its  action  on  radiant  heat,  398, 

411. 
condensation  promoted  by,  410. 

—  production    of.    consumes   heat,    172, 
210. 

—  supporting  of  spheroid  by,  177. 

—  of  metals,"  spectrum  of,  480. 

—  absorbs  those  rays  which  it  emits,  482. 
Vapours  and  liquids,  their  absorption  of 

heat  compared,  432. 

—  tables  of  absorption  of  heat  by,  375, 
380,  392,  440,  441. 

dynamic  radiation  and  absorption 

of,  392. 

Vaporous  condition  of  matter,  76. 
Varnishing  a  metal  or  feeble  gas  by  a 

powerful  one,  391. 
Velocity  of  planets  and  aerolites,  23. 

—  relation  of  heat  to,  56. 
Vibration  of  heated  metal,  116. 

bodies  having  different  tempera- 
tures, abstract  of  lecture  on,  Appendix 
to  Chap.  IV.,  144. 

sounding  disks,  264. 

Viscous  theory  of  ice,  202. 

Vital  force,  supposed  conservative  action 
of,  234. 

Volcanic  eruptions  showing  upper  cur- 
rents of  air,  188. 

—  eruption  of  Morne  Garou,  189. 
Volume  of  a  gas  augmented  by  heat,  80, 

et  seq. 

Volumes  of  vapour  proportional  to  liquid, 
table,  433. 


WATEE  boiled  by  friction,  24,  and 
Appendix  to  Chap.  II.,  68. 

—  expanded  by  heat,  93. 
cold,  93. 

—  maximum  density  of,  94. 

—  contraction  of,  by  heat,  94. 

—  pipes,  why  burst,  95. 

—  cohesion  of,  increased  by  removing  air 
from,  127,  et  seq. 

—  hammer,  127. 

—  effects  of,  when  in  a  highly  cohesive 
condition,  128. 

—  formerly  regarded  as  incompressible, 

—  Bacon's  experiment  on  the  compression 
of,  155,  note. 


Water,  amount  of  heat   yielded  by,  in 
coolimr,  I3, 159. 

—  has  the  highest  specific  heat,  159. 

—  amount  of  work  equal  to  heating  of 
1Q,  160. 

—  effect  of  high  specific  heat  of,  165. 

—  latent  heat  of.  167. 

—  mechanical  value  of  combination,  con- 
densation und  congelation  of,  168. 

—  evaporation  of.  produces  cold,  172. 

—  frozen  by  its  own  evaporation,  173. 

—  spheroidal  state  of,  176,  et  seq. 

—  frozen  in  red-hot  crucible,  183. 

—  opacity  of,  to  heat,  313. 

—  distilled,  colour  of,  318. 

—  effects  of  its  energy  as  a  radiant,  in  all 
its  states,  410. 

—  absorbs  same  rays  when  solid,  liquid, 
or  vapour,  413. 

—  amount  of,  would  be  boiled  by  the  total 
emission  of  sun,  490. 

—  cause  of  its  hardness,  252. 
transparency  to  light,  450. 

—  absorption  of  heat  from  hydrogen  flame 
at  different  thicknesses,  450. 

Waterston,  his  meteoric  theory  of  the  sun, 

498. 
Waves  of  sound,  275. 

light,  276. 

heat  and  sound,  difference  between, 

304. 

Wells,  Dr.,  his  theory  of  dew,  471.  et  seq. 
—  many  curious  effects  explained  by, 

Wiedemann  and  Franz,  their  table  of  con- 
ductivities, 228. 
Winds,  extinction  of  light  of  gas  by,  63. 

—  produced  by  sun,  186. 

—  trade,  187. 

—  direction  of,  influenced  by  earth's  rota 
tion,  187. 

—  lesser,  cause  of,  191. 
Wollaston,  Dr.,  his  cryophorus,  173. 
lines  in  solar  spectrum  observed  by, 

485. 
Wood,  bad  conductivity  of,  233. 

—  difference  of  conductivity  in,  236. 

—  apparatus  for  ascertaining  calorific  con- 
ductivity of,  237. 

—  three   axes  of  conductive    power  in, 
245. 

Woollen  textures,  imperfect  conduction 

of,  250. 
Work,  constant  proportion  between  it  and 

heat,  53. 

—  interior,  157. 


'OTJNG,  Dr.  Thos.,  establishment  of 
the  undulatory  theory,  24,  267. 


r~7EEO,  absolute,  of  temperature,  92. 
Zji    Zinc,  bands  seen  in  spectrum  of  va- 
pour of,  480. 
Zodiacal  light,  probabh  cause  of,  58,  491, 

498. 


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(510)642-6753 
1-year  loans  may  be  recharged  by  bringing  books 

to  NRLF 
Renewals    and    recharges    may    be    made    4    days 

prior  to  due  date 

DUE  AS  STAMPED  BELOW 


JAN  2  5  1996 


&PR  1  9  1996 


~~ 


20,000  (4/94) 


